I've got an X3210 in my rig......and a Q6600 in my other.....and both are B3's, and my experience has been exactly the opposite than Mark's.
Both processors, on the same mb, got to around the same OC......3.5-3.6GHz, both running at 3.2 in the interest of stability, heat production, and mb and RAM limitations.
And since both processors are essentially the same (EXACTLY the same vcore, same TDP, since both are Kentsfield and both are B3 spins....only makes sense that both would act about the same....)......they both are producing roughly the same exact performance. Of course, I have both beign water cooled......but both do run hotter for the same vcore than the dual cores they replaced.....but that's to be expected.
The thinking on Xeons vs. the desktop Q's is that since the Xeon is a server chip and more is expected from it in terms of suffering during its installed life (cramped environments that are typically much hotter than most desktop environs), server chips have, or used to be anyway, seen as having the better silicon and spin.....the better formed chips became server chips while the lesser ones were destined to become desktop chips.
Get either....you won't be too disappointed.