My kids both have trx 90's and all I can tell you is that these things run forever.
We don't race on a track but we do very agressive trail riding in Pennsylvania and it is rocky terrain for the most part.
I mean rocks, not just throwing size but basketball size.
These bikes take a pounding and hold up well.
As you know the aftermarket for the trx are endless and may cost more but I think in the endrun it is actually better cost affective.
If you get the trx you should highly consider getting rid of the stock rear tires and go with razr mx's. They make the bike much easier to power slide thru turns without the tippiness of the balloon stock tires.
Personally I am tired of 2 strokes, I know for racing they may be better but since I got rid of the 2 strokers I haven't had to do a top end in 2 years.
Me:'07 Grizzly Camo edition
Warn 2500lb winch
powermadd handguards
Yamaha rear hard box
SS108 ITP polished wheels wrapped in ITP 25" terracross tires
Stadium Shocks w/rezzy all 4 corners
Power Commander Unit
Airbox kit
EPI sport utility clutch kit
2R racing exhaust tip
2R racing foot pegs and siderails
heated grips and heated thumb throttle
My boys: '05 TRX300ex, '05 Robbs racing TRX115, '03 TTR-125, '06 DRZ-125