Both are fine choices and will work. The problem with the long chainstays is that you likely need the entire length of chain. The other thing you want to pay attention to is max cog size and chain wrap but the two RDs are probably pretty similar in that regard. Personally I'd lean to the VX GT but that's a matter of taste.

Velobase is a good source of technical info on this:


VeloBase.com - Component: SunTour Vx-GT

VeloBase.com - Component: SunTour Cyclone GT

The two RDs vary a bit but in the real world, I doubt there's much difference between them.