Personally, I haven't owned the TC5. However, the TC6 and 6.1 were made to improve a car with some inherent flaws. Looking at the two side by side you can see a drastic different. I'm not sure what your budget is, but I'd watch for a decent Tc6 for sale here with someone with good feedback.

TC5-more setup for a nimh battery... whereas the tc6 is setup for the lipos

shock towers are more adjustable on the tc6. top deck is narrower, allowing more space for the lipo. Everything seems to be moving towards a better center of gravity in the middle of the car including the chassis being narrower.

These are just a few things. You can also update the tc6 slowly to a tc6.1 over time to have the most up to date vehicle. I'm sure someone will have a better answer, but this is a start.

If you can find a TC5 in your price range with some parts, its by no means a bad option, but keep your eye open and you may find a tc6 in the same range if you're patient.