Benchmarks from CPU-World show these two to be pretty similar, are you going to overclock? Honestly a 2500k would be best with a beefy HD6850 or 560 ti graphics card.
If you are going to be upgrading a motherboard for a new processor get yourself an inexpensive P67 motherboard and a 2500k (great for gaming) DDR3 is very cheap now if you don't have it and a strong graphics card is NECESSARY.
Have a great day!!!