I agree that buying Xeon over i7 is often a good choice, but despite the similar names, the available workstation chips are not directly comparable to the newer chips. Intel's naming scheme intentionally makes it hard to decipher, but all the available Intel processors that allow more than 64GB of RAM are from a previous (Sandy Bridge/Ivy Bridge) generation rather than the current (Haswell) generation.
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.
I agree, it's certainly higher than it would be in most cases. I have the luxury of being able to design the algorithms to match the strengths of the processor. You rarely see this much improvement unless you are optimizing specifically for Haswell with intrinsics or assembly, although occasionally Intel's compiler manages some magic vectorization I never would have anticipated. And you almost certainly won't see this amount of gain if you are running an older precompiled binary.
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.