Just like everyone else I will vote for parts availability going from HPI because those are super easy to get, not only to you at your shop on the wall but to your shop in general or even online. Some of the cars just do not have a stable supply chain. Here are some that do, I think they are all equal for what you need on the track.
#1, Xray. RCAmerica is in nearly every shop you will run into
#2, Kyosho. Kyosho is orderable from almost every LHS
#3, Mugen. Mugen nearly always has stock and in every shop but maybe not for your car since people are still carrying mtx3 parts.
#4, Serpent. Serpent America under new ownership is really bumping this one up as I would have previously said this car was hard to get parts for.
I wouldn't really go with any other car because of questionable distribution channels and even the factory can be out of parts for over a month with some. Some of these cars have to be purchased with the euro making them extremely expensive as well.
Rumors, Mugen will have a new car near 2010 worlds, Xray seems due as well. Kyosho seems to be sticking with what they have had. The 733 is as current as it will be for at least a year I think.