With this new iMac and its display, the Mac Pro is starting to look a bit bleaker. I actually think it starts to look a little weird.
Performance-wise, if you configure this iMac with the 4 GHz processor, you get the fastest CPU, at least 25% faster than the Mac Pro in single-threaded tasks according to this benchmark. Mac Pro still has Ivy Bridge-architecture Xeons.
And the current Mac Pro can't drive a 5K display, but it's true that it can drive up the three 4K displays.
So the Mac Pro doesn't really make sense anymore unless you need its graphics cards to support OpenCL applications, or you want the parallelism of 8 or 12 cores, or you need its ECC RAM.