The T4 is a great car you can just pick up and run and be competitive, it's tough and parts are everywhere and there is a massive knowledge base for it. The A800X is probably a bit more advanced, needs more work continually and while some say it's more durable, I beg to differ on that, parts are harder to find and far more expensive as well. Noted weak points I have found are carbon arms in general, and the bottom chassis plate is fragile on the back where the AT21 ball stud bolts on for the rear arms, a glancing blow on the back of the chassis and it's not uncommon to just kill a bottom deck as the whole back corner tweaks. (Leverage of the arms on a single point would contribute to this as well)