I have the fitbit one. It's a pretty steep price, but for me it was definitely worth it. This is because, like you, it helps me to 'compete' against myself - if I did 5,000 steps yesterday, I want to beat that today. Also, it helps me a lot to have a goal to shoot for. Knowing I have x amount of steps to get to for the day motivates me to get it done.
You don't have to count calories at all, although they offer a system much like myfitnesspal to do so, if you choose. I personally don't utilize that. Something you should be aware of is that for me (not everyone!), when I ride in the car, it will count steps/stairs climbed. The fix is as easy as going onto their site or through their app and logging that I was in the car from time a to time b and it will un-count them, but it's inconvenient enough that I wish I had known about it beforehand.
That being said, knowing what I know now, I would have bought the zip instead of the one. The extra features on the one are nifty and all (steps climbed, sleep tracker, silent alarm), but not useful enough to justify the extra money for me.