for carpet and smaller tracks i would say stick with the Pro 4. the shaft gives ya the good torque and response you need onthose tracks. for the larger tracks belts work a little better. also if you have built you car to HARA or 05 spec the thin chassis and top plate will work great for aspahlt. all in all if your going to race small and large, asphalt and carpet, a mix of cars really cant hurt.
seeing how i dont think the pro4 is going to have a converson to the Cyclone because the cars are so different i would look into getting both if you have the money. that is what i am working on. my pro4 for small carpet, and i am either going to get a RDX or maybe the cyclone for the big out door track i race at. but i might not because i just got the pro4 and once i get some of the Hara stuff it might be a great asphalt car.
my 2 cents