Xserve vs. Mac Pro?
Seems to me you get the prospect of better reliability with an Xserve and spare parts, more (Apple-supplied) hard drive options, and Server Monitor works with the Xserve.
With the Mac Pro you get an extra hard drive bay (and therefore more RAID options?), you don't need a rack and it comes out a little cheaper than the Xserve.
Is that about what it comes down to?