E5-2570 has a 95W TDP, compared to 160W TDP for 2 E5-2407 CPUs. So, they could run up to 65W hotter. That is a tangible difference between them.
2x4C allows you to use more slots, another tangible difference (but that may not matter, since you usually just want one slot for a RAID controller).
2x4C allows for larger amounts of RAM at a cheaper cost. If you want >24GB RAM, that could be a quite a cost-saver, along with the lower cost of the CPUs themselves. There may be minor performance differences between 1x8C with 20MB cache and 1x3 RAM channels, and 2x4C with 2x10MB cache and 2x3 RAM channels, but they're going to vary by workload, and shouldn't make more of a difference than getting a higher or lower speed grade of CPU, even when they make a tangible performance difference. So, they're there, but probably not worth worrying about, if you don't already absolutely know which one you'd bet better off with (and, that would still leave the cost differences, if the added RAM bandwidth were useless, allowing you to move up a couple 2x4C speed grades for the same cost as 1x8C)
If the 1x8C and 2x4C CPU choices are similar in speed, then RAID and HyperThreading will be the hardware choices that will make the most difference, and not in a way where one choice is categorically superior to another (well, maybe using a 3-drive RAID 5 would be a categorically bad choice for a DB server )
Intel's pricing does not mesh really well with Microsoft's new licensing scheme. If you think about a FOSS server, where the software costs are either a non-issue for hardware, or done on a per-server basis, regardless of its parts, the ability to cram more CPU cores into a given U and TDP can be worth much more than if you're stuck at 8 or 16 cores, and just looking for a good way to dole them out.