I would so go with the 920 if you can afford it. 1156 is a good platform, it gives you everything from the latest dualcore to the most powerful quadcore. (the i7-860 actually beats the i7-920 in a variety of benchmarks and tests.)
It depends what you want. I think that the Core i7 860 is actually a bit more powerful, and it uses socket LGA 1156, so the motherboard for it will be cheaper than for the Core i7 920. Of course, if you plan to upgrade to faster Quad or Hex-core (six-core) processors in the future, go with the Core i7 920, since I think Intel will make all their top-of-the-line processors for socket LGA 1366, the socket that the Core i7 920 uses.
I'd rather have 920 so you can use triple channel memory. The slightly higher clock of the 860 is negligable, a small overclock will get you equal clock speeds