It is a coin flap between the two.. IMO.
i7 920 has the more stable system and better upgrade path (i9 - high $'s though). i7 860 is newer technology, uses less power and total system cost less. Performance wise the i7 860 has the edge in gaming were the i7 920 has the edge in CS4 type of applications. It is one of those debates where there is no wrong answer, since you can't go wrong either way.
With all of that said, I say go with the i7 860, unless the system is for CS4, CAD, HD Video editing.. etc, in which case get the i7 920.