In my eyes the only correct answer can be two cpus! Why that? Well, with Xeon E5 familiy you can only use the maximum of memory when there are two cpus installed. So memory is one of the points why two cpus are often better than one faster. Another characteristic of the E5 family is that the pci-e slots are controlled by the cpus and if one is missing....
And: Last but not least: In all the years i´m building, installing and supporting servers i always experince: Two cpus are always better than one if you don´t have a single task software. For example: An Exchange Server is operating better under load when it has a chance to split its tasks to multiple cpus / cores. And if you have even more CPU Cache with two cpus (both cpus you are thinking about have 15mb) you will become a better overall performance if there is enough load to split it over all the cores in the system.
If you need memory performance a second cpu would also be very useful because than you can choose smaller memory modules but more of them. Today we installed a brandnew S2600CP4 board with 2 E5-2650 and 128GB of memory using all dimm slots (so there are 16 8GB modules inside) and the system performance is just great! The machine will be used as a Hyper-V host and each of the four main vms needs for cores so we had to choose the 8core family....
Hope it helps you