Upright stationary bikes are quite similar to the dual-action bikes, though slightly fancier. And, of course, your arms stay firmly planted on the handlebars in front of you as the lower half of your body does most of the work. The computers on these bikes also play a bigger role. If you forgo the "Quick Start" button, there are usually pre-programmed cycles that take you through intervals and hills. And a button, rather than your cycling speed, will adjust the intensity. If a spin bike isn't available, I usually go for these bikes because they feel the most like a real bike.But, of course, nothing says "real bike" quite like a spin bike. They generally don't have computers, just a knob that lets you control the resistance. Kind of like the gears on a 10-speed. And the handlebars are positioned in line with the seat. Unlike the previous two bikes, you'll have to lean forward race-style instead of sitting upright. This makes it more intense, and you'll get more of a core/shoulder workout. And speaking of leaning, some spin bikes lean to the side in an effort to mimic real road turns and curves. I've yet to ride a spin bike that has this feature, but I imagine it would up the ante of an already challenging piece of cardio equipment.
Last up, the recumbent bike. If you have back issues to begin with, or if staying upright/leaning forward puts unnecessary strain on your back, this may be the option for you as it provides a more comfortable perch for pedaling. And also, a perch that's easier to balance on. Yes, you'll be able to relax, but you'll feel your abs working a little bit harder than you might initially think. And, of course, the pedaling is still a great challenge.
Ultimately, you have to decide for yourself which stationary bike is right for you. Try them all before you pick a favorite. Remember to adjust the seat correctly. There should be a slight bend in the knee when the pedal is furthest away from you, and the seat should be positioned so that your back and arms aren't straining to reach the handlebars. Proper form is essential, as it always is.
Once you've tested them all, or if you're already riding a stationary bike, do tell me which one is your favorite!