This post should be moved to either Mobile Devices or the Apple forum, but I'll bite.
The safest answer: if you want one but you're not sure which one to get, go for the Sport. You're not going to feel the twinge of regret for getting a more expensive model, and it's better for real-world abuse between the aluminum (less scratch-prone case) and sport band (more comfortable, better for sweat).
If you do go for the steel Watch, it's best to get an extra-comfortable strap like the sport band or the leather loop. The others are meant more for dressing up and tend to be more delicate.
Get a 42mm model unless you have very thin wrists.