I'm recording some friends tomorrow and I was wondering if i would be better off using a single MXL 603 for a drum overhead or XY'ing a 603 and a 604...
The big question is, what kind of music are you recording, and how big is the set? In any kind of rock music definetely use two.
My personal rule is that if I have the board space for two OH, I use two. I like the stereo spread and that is how most things are recorded these days. It sounds natural.
For what you're doing, it should be fine using two different mics (they're probably pretty close). Try a spaced pair if the XY sounds weird for any reason, although I doubt it will.