Generally speaking, the i7 860 will be faster. Although they both have the same stock speed and cache architecture etc, the i7 800 Lynnfields have a higher turbo boost than the i7 900 Bloomfields. In most single to dual threaded apps, the i7 860 will be faster, 3.46GHz if only one core is active, 3.33GHz if two. However, i7 900 use triple-channel memory, which, gives very slight boosts in certain programs.
Keep in mind that the i7 860 uses a LGA 1156 motherboard, and two sticks instead of three as they only run memory in dual channel. The i7 860 route is cheaper, and tbh, unless you're planning to do some heavy video editing or activity of that sort, the i7 860 is better. If you are planning to run two to three high end graphic cards in CrossFireX/SLI, I suggest you do get the i7 930, because the socket it is used in is LGA 1366, which for now has the X58 chipset, which has two 16x PCI-e lanes, to ensure that you do get the best out of your graphic cards. If you have three, they will run 16x/8x/8x normally. Compared to the P55 chipset, which only has 1 16x PCI-e lane, if the motherboard has two lanes, they will normally run it at either 16x/4x, 8x/8x, or 8x/8x/4x, which could seriously bottleneck some higher end cards.
If you do decide to go with the i7 860 for any reason, and want to use two - three graphic cards, I suggest you get a motherboard that has the NF200 chip, which adds an extra PCI-e 16x lane, and will make sure your cards have full bandwith.
Two examples are EVGA P55 FTW 200, EVGA P55 200 Classified and the MSI Big Bang Trinergy. Although, do note that these boards tend to be more expensive than several decent X58 boards, and even with the NF200, still perform worse than using an X58 board on average.