If you want to overclock seriously, then arguably the Xeon may be the better choice.
Remember back in the days of Socket 939, and there was a whole bunch of hubbub about whether the more expensive Opterons or the cheaper Athlon64s were better for overclocking? Well, pretty much the same deal here.
There's nothing bad about choosing a Xeon, can't go wrong with one o' them. But, do you really need the extra supposed reliability? They're higher binned chips, sure, but the regular C2Q chips are just as good for desktops, because servers just need more reliable parts.
There isn't a real reason to go with a Xeon, unless you want that fancy sticker on your case, and the knowledge that you took a chip destined for a server and slapped it into your desktop PC.
It's much cheaper, cooler and better binned than the Q6600 and weather or not a G0 stepping it's overclock will get to 3.2Ghz-3.4Ghz without a hitch(about as good as a Basic G0) and if it is a G0(Better binned), it's overclock will get to 3.4Ghz without much of a(if at all) Voltage bump and it's Maximum Overclock should be 100-200Mhz more than the lower binned Q6600.