Can we get a model number on the Xeon? Either of them will do great for the server. Having been in the server game for a while I am probably partial to the Xeon. In my experiences they have been rock solid, but since we don't have any Opteron boxes I really don't have a reference. The extra 500GB/Mo of bandwidth is really nice on the Opteron option but pretty much irrelevant for this site, since we average around 200GB total.
So if the Xeon is either the 5110 or the 5130 I would put my vote in for it, its core is built on newer tech which is along the Core 2 lines, whereas the Opteron about a 2 year old CPU.
Just my 2 cents.. would like to hear what the others have to say