I've had both Xray and Associated, and I really like the TC5, but not as much as the Xray T2 '007 or '008.
Here are some items about the TC5 I don't like:
Motor is offset approx 1/4" outward from chassis centerline. The motor motor mount on most cars have the motor mounting either equal to or further inboard of the chassis bulkheads to improve balance. Be sure you use the screw to attach the motor mount to the chassis bulkhead to prevent messing up spurs and pinions.
Wheel Bearing are slightly smaller than most cars. The TC5 uses 5x9x3mm bearings compared to the 5x10x4mm bearings. I think this was done to reduce weight slightly, but it reduced durability since the 5.9.3mm bearings are more easily damaged.
Belt tightness cannot be cured with built-in eccentrics. While the belt being tighter than other cars, I don't feel it effects the cars speed much, but it does wear out the 10x15x4mm differential bearings a little quicker.
Arm stiffness is an issue. Associated has come out with stiffer arms, hubs, and steering blocks now, but they are not part of the kit. What frustrates me is that some people at Associated stated that stiffer suspension parts were not needed...
The diffs, while very lightweight from being plastic, they have a durability issue with hot, torquey motors. Additionally, the design of the diff allows all the grease to spin right out of the diffs and collect on the outer rim of the diff or chassis. This justs collects dirt and carpet fibers. Also, diff adjustment has to be done by removing the axle to access the diff screw. For those that don't know, Xray's have externally adjustable diffs.
Now I could go on about some things I don't like about Xray, like the cost of parts, but I'm not because I break less than 1/2 the parts on my Xray than I did on my TC5.
With either choice you can be happy, like I said, I liked my TC5, but over 4-months of racing it, I found some problems that I didn't like. Since this is Associated first stab at a Belt-Drive TC, I'm hoping they improve. The TC5 will take a little more time to get dialed, but with the stiff suspension parts available now, it should improve the car on carpet/foam setups.