For a server, your only choice is a Xeon, whether the E5 2620 or some other. If AMD were more competitive, you'd have other reasonable options, but no ECC in servers that handle anything more than ephemeral network duties is just not responsible.
For dev/test boxes, the i7 has a light edge, on the face of it, but only about 15% with perfect scaling (unlikely). Also, the extra cache on the Xeon, and being able to store everything in a single computer's RAM and drive's, might help it out, assuming you aren't dealing with an easy ALU-bound problem (like scientific computations).
The dual-Xeon system would have nearly double the RAM bandwidth (8xDDR3-1333 v. 4xDDR3-1600), should that be a limiting factor.
Like has been said, though what you're doing with it matters, and it also seems like the i7 systems would be cheaper, as would 2 single Xeon servers.