If you're just starting out, or want to just mess around in the parking lot, the Sport is the way to go. I have one, and while those Pro .15's might be going out with the Indians, as Grave said, they are on hell of a motor that can take quite a beating. They're super forgiving, and if you run 'em a tad rich, they last forever. I've got one that's on gallon 18 that I have yet to have to do any major repairs to.
I actually have a Rustler that's I've done the whole gamut on: aluminum shock towers and carriers, RPM control arms, aluminum top plate, etc, that I take to the local skate park to do little tricks with. Believe it or not, I actually took the 2.5 out and replaced it with a Pro .15. The Pro .15 has a tendency to have to spool up a bit before it tranfers power to the wheels, which makes it ideal for running it on the slick coated metal surfaces that the skateparks around here use.
It kind of has the effect of starting out in second gear with a stick shift when it's snowing out. With the 2.5 in there, all that power just makes the tires spin, and it's harder to control (in my experience).
Motors aside, though, I'd go with the Sport, but one thing makes me choose the Ruster instead: bellcrank steering. It steers much, much tighter than the direct servo steering of the Sport, and makes the car feel more responsive, imo.