For rc nitro cars does a 15 or 18 cxp have more power?

Also if I was planing to buy an off-road buggy would it be better for me to buy 4 wheel drive or 2 wheel drive if I was also planing of driving it on the street. Please put reasons behind your answers!!! Thanks

if your getting a 1/8 scale buggy a .15 or .18 is to small of a engine go with a .21 and up. but if your getting a 1/10 scale buggy a .18 will be more than enough power look at this for proof

http://www.youtube.com/watch?v=OhxP_tDXzlQ