Clock for clock the SB is going to be faster so if your buying right now then the 2600k (or 2500k) would be the best choice. They are also on a new socket that has some hope of seeing new Ivy Bridge chips down the line, though don't buy it as that may not pan out. Buying an i7-9xx would not be smart unless you need to have tri-sli or triple channel ram as the 1155 socket does not support those but the 1366 socket does.
If you already have an i7-950 then getting a 2600 does not make much sense. Yes, the 2600 will be faster, overclock higher, and run cooler but unless power usage is a huge concern the difference in performance will be noticeable only on benchmarks. For "real world" use there is not much benefit to buying a 2600 if you have an i7 currently.