Educate me, oh wise one.
The Mac Pro gives you more cores, more drives, more ram, and is more expandable.
Only as an individual unit. Taken all told, an xserve at 1u leaves a lot more space on the rack for external arrays, proper switches, and, of course, more xserves.
The MP is *not* designed for a datacenter, no dual power supplies, no externally accessible drive sleds, too much rack space. Even that expandability is ridiculous. In a data center you often have 3 types of machines (well, only talking about servers here, ignoring disk, network fabric, etc for a sec) these days: blades in chassis (apple doesn't make anything in this category, never did), 1 (xserve) and 2u servers, and the occasional 4-6u server (mac pro in this case). Those 4-6u servers though tend to hold *lots* of drives, *lots* of CPU power, and a helluva lot of ram. The MP is not designed to play in that arena.
In other words, yes the MP is more powerful than the xserve, individually, but *not* the way a machine room usually is deployed.
But can the OS or the applications that run on it, really make use of 64GB of RAM?? Apple spin, doesn't make the computer any more productive.
On the server side? you better believe it!
I run jobs on machines with over 100GB of ram, and can easily eat it all on the right job, and that's in scientific computing (MD). If you look at DB servers, or large application servers, they can eat even more- easily