3g or Wifi model? Pros/cons to each different version?
With the 3G version you can use your vita's net features out and about. It isn't a contract based service, you just gotta make sure you don't have the auto pay for next month option selected. If you're preordering, you get a free pass that you can only activate after paying for a month but you also get a free game, super star dust delta and a 8gb memory card. It's all for 299.99, 50 bucks more.
If you want a discount then go for the wifi version if you're used to only connecting online in the methods that the psp and the 3ds portables have done so before.
Personally i'd go for the 3G model. You never know when you're on a trip or the net goes out or something right? I have a trip to my gramps coming up and his network really sucks. so i'll definitely make use of the 3G feature.
3G
Pros: extra features and mobility, more beneficial pre-order bonuses
Cons: more expensive for features you probably may not use.
Wifi
Pros: cheaper, use that extra 50 towards a game.
Cons: no mobility features, stuck on relying on a hotspot or wifi connection.