The i5 2500k has an unlocked multiplier for easier overclocks along with a better on die video chip compared to the i7 2600.. The i7 2600 has only HT as an exclusive feature.. If you do not need any of it (HT or overclockability or better on die video chip) then go for the i5 2400.. Save cash to invest on some other component..
Scenarios wherein you won't need all or either of those features -
1. HT - not required for gaming and/or general desktop use..
2. Overclockability - not required if you have a mid range single GPU configuration in which case, turbo clocks will be plenty..