What type of driving do you plan on doing? If you're looking at the Nitro Sport, I would suggest spending a little more and getting the Rustler. The Rustler has a full aluminum chassis where the Sport is half aluminum & half plastic. It will also have a stronger engine and better steering setup. Other than that they are very similar, and parts are interchangeable. I have both and I use the sport as an on road car, while the Rustler gets 10' in the air off of skate board ramps.
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I love my Rustler, a lot of people will say it's not very capable but I have a blast with it and it never lets me down. Throw RPM arms, towers and bumper and it can handle some serious abuse.