Xeon's come in multiple cache variations. The bigger the cache the more expensive the processor is.
Essentially the two chips are the same (reg. P4 and Xeon) except the pin count is different and you need the proper motherboard for each. Also with the Xeon you can run it in either a dual and quad configuration whereas with the P4 you can only fly solo. Although VIA is rumored to be working on a dual P4 mobo. Not that i am a big fan of VIA chipsets but it should be interesting nonetheless.
The Xeon would probably be your better choice because you can pop in an additional cpu in the future and almost double your final rendering speed. Your viewport speed is more a function of your graphics card than anything else. I think the only part of Maya that is multithreaded (able to take advantage of more than one cpu) is the renderer.