The Xeon IMHO is the better deal of the two hands down. However, if you want to get both, Opterons (G34's) are still nice to run here if you can afford the electricity. The AMD chips had poor design for things like AVX and thus the Intel's blow their doors off when apps can fully utilize those optimizations. WCG doesn't usually have optimized versions of the apps, so don't normally come into play here much.

CPU's only... http://www.cpu-world.com/CPUs/K10/AMD-Opteron...20(OS6128WKT8EGOWOF).html

http://www.cpu-world.com/CPUs/Xeon/Intel-Xeon%20E5-2670.html

80-115 Watts each chip. Then the RAM you use, HDD, etc...

The good news is that G34 CPU's are really cheap. Check the board he has and see if you can put some better chips in it. I have a couple of 6274's in my G34 2P which are 16 cores each. It has been turned off for a few months as I need a new PSU for it. Some of the 12 core Magny-cores are usually 12 core chips but many find them to overclock better. So, depending on setup and project.... If you aren't going to overclock, then I would look for some cheap 16 core chips if your board supports them. The good news is that you can also use non-ECC DDR3 with most G34 setups.

Now, if you were a member of team HardOCP, I possibly could have access to a few http://www.cpu-world.com/CPUs/K10/AMD-Opteron...20(OS6172WKTCEGOWOF).html but doubt he would want to ship outside USA due to the cost of shipping....

You will have to look at the motherboards specs to find out if ECC is required. AMD is typically less restrictive in that area.

The e5520's are only quad core with 8 threads. Those use socket 1366 which is older (first iSeries chips) than the previously mentioned 2670's which were socket 2011. Socket 2011 motherboards have held their value quite well. A lot of the 1366 stuff only had ready boost on one of its cores whereas everything after I believe was all cores. I could be wrong on that. I would not waste much money on anything older than the socket 2011 stuff.

The e5-2620 is a hex core with 12 threads. It would certainly put out more PPD than the e5520 and would be a socket 2011 chip as well.

My advice would be to go with the socket 2011 stuff before the 1366 stuff if you are thinking just WCG as the more cores/threads will more than likely give you better output than the higher clock speed with lower threads. I don't know what kind of pricing he is giving you but definitely compare rates on eBay (that are actually being sold and not what is being asked). With tech that old, I would stick to dual or quad socket setups. Otherwise you are probably wasting a lot of money. The appeal to these older setups is that DDR3 RAM is really cheap compared to DDR4. DDR4 probably won't gain you much anywhere but the Math BOINC projects. Ryzens are great chips, but hard to justify the up front costs when there is so much decent hardware from previous generations being wholesaled by corporations cycling out their equipment.

I agree with Coleslaw. I run 3 dual socket systems with 1366 sockets with hex core cpus. They do use quite a bit of electricity compared with the newer 2011 socket machines. Any new generations will subsequently be more efficient.The saving grace for me is I got the systems dirt cheap, $50.00 apiece.
One thing to beware of if you are interested in any quad socket or above machines is to check their total power draw at the wall under load so you put them on an appropriately sized circuit. I passed on a quad socket machine for that very reason.
Cheers