Well it depends on what you want to do with it, and how much you are willing to spend. For things such as gaming and "normal" use, you will be much better off with a socket 1155 combination (2500k/2600k) but if you are doing things such as heavy video editing and other such "professional" tasks the 2011 socket may be the better choice.
It depends on what you define as 'better'. The X79 allows up to 64GB or RAM, up to 4-WAY SLI/CF, the SE-E i7-3930K is 6-core + 6 Hyper-threads and outperforms the SB 4-core + 4 Hyper-threads (2600K)/0 Hyper-threads (2500K).
its ALOT better for all things, i have the new X79 Rampage 4 Extreme with the 3930K and i used to have the 2600k and the new gear is much better, not that the old was shocking or anything but all new gear WWILL be socket 2011 so no point buying old tech just get the new
X79/SB-e is Intels way of giving you everything AMD does but for 10-15x the price. Just not worth it if you are not wealthy.
If you want to play games go 2500k with asus z68 board
if it's for business ,or trans-coding videos 2600k
2600 k is more a mobile part bin than game machine though it's potent enough for games the 2500k trumps it.
the new isn't worth the cost as far as I am concerned unless you have money to burn .
Is the i7 3930k with p9x79 better than a i7 2600k or 2500k with p8z68 ?
Based on performance, yes..
Based on price/performance, no..
Based on performance/energy, no..
If you use to render 3D images, video editing, yes..
If you use to play game that things too overkill..
If you use to convert your favorite movie to the other format, no..
The last thing is that if your daily use only to watch movie play game, convert you favorite movie it would be wise if you choose the 2nd..
But if you use your system as workhorse to render high polygon graphic, edit a high definitions video or overclocking until its limit then you will be satisfy with the 1st choose
all new gear WWILL be socket 2011 so no point buying old tech just get the new
Socket 1155 is still very much alive and kicking. Socket 2011 is not replacing Socket 1155, its replacing Socket 1366.
Socket 1155 is geared more towards your average home-user. Maybe some light business users. IvyBridge CPU's will be both Socket 1155 and Socket 2011. http://en.wikipedia.org/wiki/Ivy_Bridge_(microarchitecture)#CPU_specification_comparison