The LGA 1366 socket is somewhat obsolete considering the LGA 2011 (it's newer successor) is to release somewhere in November of this year. The LGA 1155 is a much better choice simply because Sandy Bridge offers tremendous power and has an upgrade path to Ivy Bridge.
The 2500k does not have 1/3 more power nor is it obsolete in any way.
Sandy bridge is clock for clock better than the older nehelams by 10-15%. Although it is slated to be replaced by LGA 2011 next year by SB-E, the fact is that it's still a top notch CPU and if you find it for a good price then by all means get it.
But yes the 2500k is better by 10-15% clock for clock but neither is slouches.