If these were my choices it would be a no brainer. The i5 4670k.
The FX 8350 is beaten by the i5 in most tasks, and only in the most heavily threaded applications does it win. In single core applications (and 90% of what an average user does is single core), the i5 Haswell part smokes the AMD processor.
The graphics card you have will play a bigger role than the processor in gaming (so get a good card), but the i5 4670k is on a new socket, and has a very decent upgrade path that will be good for years.
there are quite a few Videos on Youtube about pitting the FX 8350 vs i5 3570k...And most of them will tell you that in games there is not much of a performance difference. Like in a BF3 Vid i saw the performance was almost identical using an HD7970 with the first two missions. I would say based on my somewhat limited knowledge of AMD that in most cases the i5 4670k would be much faster. the Haswell architecture is much newer than the AMD FX 8300 series(piledriver) and it shows in synthetic benchmarks. You can use those as a guideline to how well your CPU will perform in real world applications. For current games the i5 4670k will get a few FPS more seeing how it is about 5-7% faster that the i5 3570k. But take this with a grain of salt as AMD's new API "Manlte" is still a factor for future titles.
from the newest Ubisoft AAA game called ''Watch Dogs'' scheduled to come out on may 2014 and looking in the future, i would say FX-8350...
looking at the PC requirements for this game, the Recommended specifications are as follow :
Processor: Core i7 3770 @ 3.5Ghz or AMD FX-8350 @ 4.0Ghz
RAM: 8 GB
Video Card: 2048 VRAM DirectX 11 with Shader Model 5.0 or higher (see supported list)
Sound Card: Surround Sound 5.1 capable sound card
Supported Video Cards at Time of Release:
nVidia GeForce GTX460 or better, GT500, GT600, GT700 series;
AMD Radeon HD5850 or better, HD6000, HD7000, R7 and R9 series
Intel® Iris™ Pro HD 5200