OK, I don't know whats different between the RTR Sports 3 and the US Sport, but I can say:
The US only has one set of spider gears per diff. Costs around US$20 to add the remaining gears.
The US only has one chassis support brace which is in the rear. I've taken some pretty nasty spills and haven't had any issues.
The US has center dogbones but not CVD's.
All the diffs are filled with grease instead of silicon oil.
The engine is not SG, so you will need to buy a new clutch assembly in the event you switch engines.
The radio is really good, but be prepared for interference from just about any AC source as its AM.
The front C-hub limits steering travel. This can be overcome with a dremel
The front C-hub does not have a shield so it's easy to get dirt into the wheel bearings. There is a 20deg replacement which fixes this. Don't forgot to get the matching a-arm.
The steering servo connection is a bit flimsy (the rod between the server-horn and the bellcrank). I've yet to replace this on my dads, but I can imagine a hard hit would break it or knock it loose.
Overall, this is a great starter buggy. If you're the type that has to upgrade everything (as I am), then you will probably want to start with a roller and add your own electrionics and engine.