The Workstation Xeon parts are almost identical, and allow much more ram, and don't cost a whole more.
The clock speeds are similar, but there are lots of differences under the hood. Whether these matter depends on your use case, but in many situations with integer compression algorithms, I'm finding that I can get 50%-150% better performance per core with Haswell. Partially this is due to the AVX2 instruction set (which adds integer operations for 32B vectors), but more than I'd expected this is due to the BMI/BMI2 instructions and improved memory throughput.
[1] http://www.chiploco.com/intel-haswell-ep-e5-1600-v3-35072/ [2] http://www.cpu-world.com/news_2014/2014080502_Xeon_E5-2600_v...