There was a rumor a while ago on OC Forums that clock for clock E0 might be slower than C0 steppings. I finally found a thorough review that shows when both processors are at the same MHz that there is virtually no difference in performance. Any slight difference favors E0. The only real difference is the E0 runs the same MHz at less voltage and when you start overclocking, the E0 has a lot more headroom.

http://www.xcpus.com/GetDoc.aspx?doc=49&page=1