It depends...
When you combine mobo with processor costs (IF you can buy at microcenter), I don't think there's a huge price difference.
Advantages to i7-950/X58:
- 6 Dimm slots (more ram, cheaper)
- Triple channel memory (for some things, only a marginal improvement)
- EASY, fully supported install of OS X (This is very important at the moment)
Advantages to i7-2600K
- Faster clock for clock (at least marginally)
- Runs cooler
- Easier to overclock to higher speeds
The main thing about 2600K is that until Apple releases Sandy Bridge builds... going this route is a bit of a science experiment. So don't expect 100% compatibility/stability.