what is the difference between a 190mm and a 200mm touring car, is it the width or the wheel base.

That's the width.

190 is electric and 200 is nitro

There ARE or Were 200mm electric cars and/or options to make them 200mm (like the Rally Kit for the TC3 and TC4s

I know some guys make their TC3 and 4's 200mm to run on the DIRT ovals.

On the Vintage Trans/Am cars, we use 190mm cars, but 200mm VINTAGE bodies w/ the 31mm rear wheels, which kicks the width perfect to fill the wheel wells.