It might be interesting to slap a PPC Linux on the Xserve and do a bake off. OS X has some deep-seated (and well documented) architectural issues that make it a *lousy* server platform. (For instance, "fork" takes four times longer than it does on Linux and its threading performance is similarly lousy, leading to a situation were programs like MySQL scale approximately *eight times better* on Linux than OS X on the same hardware.) If your benchmark is Netburst-based hardware the Xserve just *might* pull off a win.
Granted, life is probably too short to waste your time on things like that. (Although there are some perfectly stable/boring distributions like Debian that still have PPC versions and should work just like Ubuntu as long as the software you want is available in a Debian repository.)