The bicep curl machine or bicep curls using a set of dumbbells? The leg press machine or squats? The great machine/free-weight debate is perhaps one that will never end. Some swear by machines, while others avoid them. Why? The reasons, like the debate, are endless. And different, depending on who you talk to. If you were to ask me (we'll pretend you did), I'd tell you that free-weights are better. BUT—there is an appropriate way to combine both in your fitness regime. Variety, after all, is the spice of life.
If you are a beginner, I would recommend that you start with the machines as you venture through the learning curve. The machines will assist you with proper form and help you figure out your body's mechanics as they relate to exercises for specific muscle groups. But the machines do, generally speaking, only target specific muscle groups. That's why I prefer free-weights (and other toys like balls and bands). Even when I'm working with clients.
For example, in the case of the bicep machine, your biceps work especially hard with just a smidge of help from your shoulder muscles (which act as stabilizers). But as you progress in strength and comfort, if you were to stand up and curl a set of dumbbells, you'd rely on those shoulder muscles a little bit more because there wouldn't be a pad underneath your elbows into which you can dig, and your body (not the machine and it's fancy cables) would be in total control of the weight you've chosen to lift. In addition, your core muscles would be working overtime to keep your entire body balanced—which definitely isn't a factor while you're SITTING at a machine. But, like I said, sometimes boredom hits and it makes perfect sense to set the dumbbells down in favor of the bicep curl machine. Or any other machine, for that matter.
In the end, it's all up to you. Despite the fact that I prefer free-weights over machines, you have to do what's best for your body, mind and soul. And I'm curious—what is best for your body, mind and soul? Do you prefer one over the other, or do you mix it up? Not sure, send me a note. I'd be happy to help.