The TX (and identical motor T300) uses a brushless motor, compared with the more traditional brushed motor in the T500.
My comparison testing between my T300 and T500 using iRacing wheelcheck application showed that the T500 is about 25% faster in terms of spin speed. But as that link above explained, you have a lot more damping on the T500 and it does indeed feel slower to react. The higher power motor combined with more damping makes it harder to control cars, and you can definitely hold slides easier on the T300. The T300 is also clearly smoother due to the brushless motor, and probably also due to the better belt mechanism which has lower drag.
That said, the T500 is more of an "event" to use, since it's more brutal, needs to be worked harder, and the self-aligning torque replication is more powerful meaning less to do when drifting (you almost never need to help the wheel around, whereas the T300 is just about on the edge of needing to be spun manually to initiate a drift - in comparison the G25 needs to be constantly helped when drifting, since it's far too underpowered). This means catching a T500 while drifting takes skill
I prefer my T300 for the low damping, seemingly lower latency and ease of use catching slides. But I know some people would prefer the more brutal and possibly more realistic T500 experience.
I should point out that the TX/T300 electronics (to support that brushless motor) and power supply is the weak spot and has caused quite a high failure rate. In comparison, the T500's big chunky brushed motor and external power brick is solid and unlikely to give out on you.
