Earlier last week friend of mine asked me the question in the title. I composed pretty lengthy response to his question to help him decide. It summarizes my experience with both HRMs. After reading it again I realized that there may be other people with similar question. Perhaps this one user summary will help you decide what to do. Please consider that I'm a triathlete and may have a different requirements from my HRM than pure runners (main being water resistance and second the support for multiple speed/distance sensors for tri racing). So here you go:
So you are on a quest as well? I just went through a number of HRMs and plan to sell some on eBay in the next few weeks. As for the 405 vs. T6c I think I can give you my take as I own and use both at the moment. It really depends on what you intend to do with the HRMs so I'll look at them from few angles.
Garmin Forerunner 405 - Garmin has some planning functions (limited compared to RS800 from Polar), but you can structure the workout as you need which is quite nice especially for complex intervals training if you need to do it on treadmill. The result of planning is a guided workout in the watch that you can assign to particular day and the watch will let you follow it when you are ready to train.
Suunto T6c - has some planning capability in the software, but the guided workouts are to be defined directly in the watch. You can define warm-up duration, countdown duration (in case you want to do warmup, but not include it in the total exercise time), then there are two time intervals or you can use distance intervals (again only two). I find this fairly good for training and less robot like training - at times with the RS800 I felt like a robot executing a workout. The T6c gives you little more flexibility to adjust the workout as you feel just before you start. Very similar to S625X from Polar in terms of interval workouts.
Garmin Forerunner 405 - the Garmin watch is great for running, OK for cycling and totally unusable for swimming. During running it connects to GPS very quickly and keep the connection just fine (despite the smaller size compared to 305). You can also use the foot pod (additional 99 USD) to keep track of your cadence during the runs. If the watch loses GPS connection it switches to the foot pod. You will need the foot pod on the treadmill if you use it. For biking you can get the bike pod, but I do not know much about it as I do not use it. The HR belt is fairly comfy, but it is a big rubber band like the very old Polar watches, but it is more comfortable. If you do not use HR belt Garmin approximates the calories expenditure, but the algorithm is weird - it told me my 56 miles bike ride burned 4500 calories. Yeah right. One nice thing is the GPS navigation - you can actually build track in the Google Earth or on the web like mapmyrun.com and transfer it to the watch as a course and then execute it. You can also take advantage of navigation if you do out and back run. The watch will guide you back to your starting position (through all the places you ran before) - basically backtracking home.
Suunto T6c - works like a charm for any sport and automatically switches between sensors (pods) if you use multiple - bike pod, foot pod, GPS pod. I use the watch with Memory Belt during swim and with comfort belt for running and biking. The comfort HR belt is very nice - even better than the Polar WearLink (the new one) and works very well once you figure out which part to moisten. The watch has configurable screens - 2 screens 3 lines each and the last line can show multiple items that you can switch between. I like it more than the Polar approach or Garmin approach (very similar - 3 items on each display and you switch between displays). What is really cool the watch switches between the sensors automatically. I did a brick today and it recorded my bike speed / distance (as 18mph), then I did a lap for transition and headed out of the house for a short run. Before I hit the street it was already switched to my foot pod and was showing my pace 18:30/mi and soon 7:45/mi. It is pretty cool to see this working after struggling with this with Polar, previous Suunto and Garmin. This may not be as important for one sport athletes, but quite nice for tri-geeks...
Evaluation of training
Well compared to Polar software both Garmin and Suunto have a long way to go.
Garmin Forerunner 405 - The evaluation software is pretty basic, but it has a nice feature of loading the GPS data and showing you the map of your run. It is nice, but to some degree it is a gimmick. It wears out pretty quickly. I mean how much do you need to know about where you ran around your house. It is nice though for all the business trips. Other than that the data is fairly basic and the data has the usual issues of the GPS sensor - the pacing data are fairly useless (quite an oscillation due to fluctuating GPS signal and the fact that the resolution is about 3-5 meters which does not really work well with immediate pace), I have very little confidence in calories burned. The HR data is very basic - no beat-to-beat info like in Polar RS800 or T6/T6c. Nice thing is that you can set your pace zones and HR zones and see how you did during the exercise. Overall I use the Training Center very little - mostly to load the courses or review the high level data.
Suunto T6c - the biggest difference between T6/T6c and other HRMs is the EPOC and TE. The standard STrM software does the basic analysis and shows you the data recorded by the watch - R-R data, HR, speed / distance and all the parameters the software calculates based on the R-R. I use also the FirstBeat Athlete software mostly to help me look at my EPOC load during the week and month. I do get variety of the TE and EPOC based on how hard my training was, but it took me a while to fine tune the parameters. There are very good guides in the Wriststop Trainers group on Yahoo!. I fine tuned my parameters after few runs and Cooper running test. Since then my EPOC and TE makes a lot of sense and reflects how I felt during the training. I do review the detail mostly HR, R-R, EPOC and TE in both STrM and FirstBeat. FirstBeat seems to be more precise than the STrM especially for training of TE 1-2. For TE 3-4 the results are very similar. The T6c now shows the EPOC and TE during the workout and that is good, but not ultimately how I regulate my workout. I have my own training plan that I follow and use the EPOC / TE for overall evaluation at the end of the week to make sure my easy week is easy and build week is at least rated as moderate in the FirstBeat.
Daily use as a watch
Garmin Forerunner 405 - the watch can be used as every day watch, but it may be too large for people with small hands and people that need to wear shirts. It does not fit easily under the sleeve as it is quite bulky (the GPS needs to go somewhere). You will need to charge the watch every 3-4 days to keep it going. It has about 2 weeks of life with no GPS use and about 8 hours of continuous use of GPS. It is quite nice and the design gets you some attention. I heard people commenting on the watch and expressing interest to get one as well (mostly people doing some exercise).
Suunto T6c - It is a watch that you can use daily. It fits under the shirt and goes with any outfit. Since I got this watch it is my only watch that I use every day. I take it off only when I shower.
Note that this section reflects the situation as of June 2008. The prices are likely to change over time so do your research, check sites that sell Suunto watches and are certified by Suunto (you may not want to take chances with buying new watch from eBay and losing the warranty - I believe Suunto does not honor the eBay purchases. I actually called their support line before I purchased from Amazon to make sure they will honor the warranty).
Garmin Forerunner 405 - the watch is now about 350 USD + tax, foot pod about 99 USD and the bike pod 60 bucks
Suunto T6c - I got the watch for 320 USD (I guess I was lucky as the prices are back up to 429 USD on Amazon), foot pod for 90 USD and bike pod for 50 USD
So what it means for me - well I will continue to use both. I like the Garmin even with its shortcomings for recording speed/distance and position during running and biking. And the T6c is my primary HRM. Sometimes I run and bike with both, but most of the time you will find me with the T6c.
I do not have a simple answer T6c or 405, but you may be able to decide by reviewing my perceptions above and evaluatin what is important for you. I feel like I could live without the 405, but like to keep it as a second HRM.
If you want to research this topic even more there is a discussion thread on Suunto Discussions where I shared some more thoughts about the Garmin 405 vs. Suunto T6c more from a runner's point of view.