It all depends. Evo is slightly cheaper (plan), but with that said, im sure the Droid X is way more powerful. It has a better processor (face it guys, qualcomm always sucks) and a dedicated gpu.
Oddily enough, i think the evo's aesthetics are well enough worth keeping it. To me, it looks way better and also has that kick stand that i didnt think i'd make so much use of lol. It really is an awesome phone by design if not by hardware (and battery life
)