I ride clipped in because it's what my body is used to. I've been clipped in to every bike I've owned since '95 or-so, so it just feels wrong for me to not be.

My feeling on the subject, though, is that you simply can't get it wrong. Large platforms have the obvious advantage of day-to-day simplicity and many more low cost options are available. SPDs have advantages of, as was mentioned earlier, the option to recruit muscles that simply cannot be used if you're not clipped in. (Your tibialis anteriors will grow to the size turkey basters!). Also, although less of a concern touring, I actually feel safer clipped in. There's no chance of my foot slipping off a pedal and I like that feeling. I use SPD shoes that wear like regular sneakers when I'm touring, BTW.

But really, if you don't know for certain what's better for you, just choose one. You can't get it wrong.