I'd say go with the rusltler, why you ask?

1) it's a better known car, people know more about it therefore more people can help you with it
2) the engine is better (faster, you can still get parts for it)
3) the rustler has a lot of aftermarket hop-ups, I don't know if the sport has any

Traxxas made the sport with the beginner in mind (it's not for an experienced hobbyist, less adjustments etc), but when they made the rustler they thought more about an experienced hobbyist so they made a better car. Go with the Rustler it would be the better car. By the way on www.towerhobbies.com the rustler is only $50 more.

But the Associated rc10gt is a really good truck too, you might want to check it out. They also just came out with the gt2 which in my opinion is better than the rustler and the sport. The gt2 is raced by pros today. It has a lot of hop-ups available and is backed by a great company (Traxxas is an awesome company too).