What are you using these for? That is a pretty important data point for this discussion.
Sorry yeah should have stated. It's going to be my home server... replacing a Xeon X3440 16GB (maxxed out) setup... just really want something that will last me a while and has a lot of expansion, but something previous gen so it won't break the bank. So I picked up the Supermicro X8DTH-6F for $250 (pretty good deal I think, considering everything that board can do!) and a second-hand SM SC745 chassis. I've got 48GB of ECC Registered DDR3 ready to go.
I run (and will run) ESXi 5.1 and will use Vt-d to some extent.
My current VMs are somewhat memory-starved rather than CPU starved:
Pfsense
OI/Napp-it
Win7 x64 for media center (I use extenders on the TVs for viewing)
Windows Home Server 2011 to back up my windows machines
OS X Mavericks for my main iTunes & Aperture libraries
Couple of small FreeBSD VMs for various tasks
With all that (and due to memory reservations because of vt-d) my current server just can't do much of anything else. So I'd be running all those VMs, plus I want to prop up some Win8.1 VMs for testing, maybe a couple Server 2012 instances as well. I also want the ability to do some multi-threaded video encoding & transcoding (which is the main reason I'm looking at X5650).
Mainly I guess I my goal is to oversize just a bit so I'm not right back in the same situation in another year or two. Granted, the additional RAM capacity I'm getting with this setup will be the biggest factor but at the same time I don't want to feel like I took a step back CPU-wise (not everything I run will benefit from multi-threading).
It looks like I could get a pair of L5630s for <$130 or a pair of X5650 for <$250. Just trying to decide if the extra four cores and higher clock speed are worth the extra hundred bucks and 110W (combined) higher TDP (considering they will be idle most of the time).
I realize there's no one "correct" answer here. Just wondered what others would go for in my situation. Thanks Patrick and et al.
Edit: I should add that power consumption is something I care about also (I know I'm already jumping way up in that department from the X3440 to any DP Westmere setup). In the real world, with most of the cores idle I'm just not sure how many watts difference I would see between 2x L5630 and X5650. If it was like 20W or less that's probably trivial, but if it was likely to be >20W then that would be another reason to go with the L5630.