The Answer honestly depends on the budget and what kind of applications you want to run.
If you have around 17K for CPU+Mobo then go with the i5 2500K and P67 motherboard. If you are on a tight budget, then you can get a Phenom2 955+AM3 motherboard for around 12-13K.
If you are into photo/video editing, 3D rendering, audio editing etc, then don't opt for AMD.
For gaming, if you play RTS like Warcraft, Starcraft, Supreme Commander etc, which need powerful CPU for AI, the i5 2500K beats the AMD CPUs by a very large margin.
For other games though, the differences are not so stark, and you can use the money saved to invest in a higher end GPU. The difference is enough to upgrade from a Geforce 560 to 570, for e.g.
Overall though, for performance, efficiency and value for money, the I5 25xx and above are the best choices, but for people on tight budgets, AMD offers very good options.
Think and Decide
---------- Post added at 02:13 PM ---------- Previous post was at 11:26 AM ----------
And honestly, 80% of the software out there (and 90% of games) don't use more than 2 cores, even lesser use quad cores. So 6 cores might sound more future proof, but remember that quad cores have been around for a while now.
by the time software starts making use of 6 cores, there will be much better CPUs out there. so for the next 2 years at least, get the best quad core that can fit in your budget.