If it's one musician solo, and you can run onstage right in front of them, then I second XY all the way. Also, if it is only one sound source (solo instrumentalist), I might even run the XY a little tighter than 90 degrees, say 70 degrees or so. The reason is that with a full 90 degrees, you're really only picking up the main sound source off-axis to the cards and the by summing them the image places the source in the middle of the image. Thing is, in this case, then you're only getting your main sound source on the off-axis part of your mics, which can be colored and generally not as good as straight on. And in the case of one instrument, you aren't going to be getting much of a stereo image anyway, so I'd pull in the angle a bit so the mics are picking up the sound more in there sweet spot. Anyway, that's my two cents....