the i7 860 has higher turbo boost and is a little bit faster, but if u want a futureproof system the x58 platform (i7 920) offers more advantages than the p55 platform (i7 860)
The 2.66ghz one is based on socket 1366, which is better than the 1156 socket of the other one. In reality though, the 1156 one is usually better because of turbo mode.
the i7 860 has higher turbo boost and is a little bit faster, but if u want a futureproof system the x58 platform (i7 920) offers more advantages than the p55 platform (i7 860)