Firstly, I am not an Intel or AMD fanboy -- so keep that in mind.
Buy a 2500k
1) For gaming, the difference between the two are negligible, as most games rely on GPU power, nevertheless, the 2500k shows improvements in games that are CPU -intensive because as far as raw CPU capabilites go, Intel beats out AMD by a pretty good margin.
2) If you are a grahics designer, 3D modeler, AutoCADer - for basically any CPU-intensive application, the 2500k beats out the AMD FX once more.
Although AMD put lots of emphasis on thi new 8 core stuff, the performance does not translate directly for two reasons.
A. Each of AMD's 8 "cores" are not allowed the same amount of resources as each of Intel's 4 cores.
AMD didn't put 8 full cores, instead, they have two "modules" each sporting 4 lesser powered cores that share resources, thus, 8 core. The trade off is that you get less performance per core and that is very noticeable in most programs.
B. Most programs are coded for 4 cores, so although you are getting 8 "cores" from AMD, not all of them will be used to their upmost potential, because the program is not written to distribute the workload between 8 cores -- few programs are nowadays.
Benchmarks have showed FX processors paling in comparison to even the older AMD 6 core 1090t, a generation before. Furthermore, benchmarks have showed the 2500k wiping the floor, basically, with the FX.http://www.anandtech.com/bench/Product/288?vs=434
So in conclusion--
If you are a gamer looking to buy a good CPU to play the latest games, opt for 2500k
If you need a high-quality, high performance CPU - opt for 2500k.
At the same price-point, it becomes common sense.