.21 engines have less rotating mass and less surface area on the piston and sleeve so you get less friction... this allows .21s to get more top RPMs and for the RPMs to come up much faster. I'd sujest getting the Sportwerks .21. It has LOADS of bottom end power with some great RPMs and it wont put a huge hole in your pocket at only $180 compaired to some of the higher end engines.

Edit: The bigger displacement engines like that .28 give you more torque but not more HP or rpms. In a buggy its mainly to get more sales to the people who want something bigger and more powerfull. The big displacement engines are a must to have in monster trucks though because the .21 just don't have the torque to carry those heavy trucks.