RE: What a tough desicion!
Well if you're thinking of the Revo, then you probably won't want to switch the engine out. The TRX 2.5 is a powerful small block and will contend "as is". You should be happy with the size engine that cames in whatever truck you choose, because upgrading to a more powerful engine often times means upgrading the drivetrane as well, which can get pretty costly. I put a Sirio 18 in my Tmaxx, and had to upgrade the entire drivetrane, because it was too powerful for the truck. The Revo is designed to handle the TRX 2.5R and nothing more.
By getting a small block truck, you are limited to just that, a small block truck. While they can be great, your engine options are limited without costly conversions and upgrades.
Although the Revo is impressive, if you want to race you should give serious thought to the GS Storm SUT, as it is a pure bred race machine based off the buggy platform, and includes a .25 big block engine, which means your future engine options are wider.