I've not seen benchmarks, but I'd guess the Xeon would get more work done (at least with SMP apps).
Have a look here: Passmark benchmark results
Specifically look at the systems on position 10 and 16, both Xeon's and the rest that are almost exclusively i7 systems.
You will get more bang-for-the-buck with an i7 system.
Specially the first guide will give you info you may need.
Read the whole thread.
When I ran the PPBM+ test for CS4, that Bill Gehrke is currently testing and preparing for wider use, Bill remarked, on seeing my results:
Phenomonal results as I expected from your setup.....
Makes your decision to go single i7 overclocked processor an ideal and extremely cost effective solution!
If you have studied the PassMark results in the previous links, you see that even dual W5580's have trouble keeping up with a number of i7 systems. When you take into consideration the additional cost of dual W5580's versus a single i7 (2 x 1380 euro versus 220 euro), the higher mobo cost, the more expensive memory and the limited OC capability the most economical solution is an i7 system without taking a performance penalty.
Another consideration is that the dual Xeon's use nVidia Quadro cards and Bill found in his testing of the PPBM+ testing suite that the Quadro cards carry a large overhead, which seems to be confirmed by these PassMark results. You mentioned you will be using the Quadro CX, which may well be hampered by the same behaviour. In that case a look at an ATI 4890 may pay off significantly, 210 euro for the ATI versus nearly 2000 euro for the CX with the plug-in, because the benefit in performance will at most be negligent. You may be better off using the savings with another video card for improving your disk setup. That will benefit all applications, not only H.264 encoding.
My complete system set me back slightly less than 3000 euro, which would not be enough to get dual W5580's and a mobo. For that amount you would have to run without a case, PSU, memory, disks and video card, kind of difficult....
At different OC frequencies my system occupies place 3, 9, 14 and (not-included in this listing with a score of 4033) 15, the latter @ 3.6 GHz, moving the second W5580 system to place 17.