I'm working on a purely gaming only build for a friend.There is literally no budget and whatever is bought will be paired with a Vapor X 7970. Which would be the ultimate gaming build? i5-4670K mounted on a Asus Maximus VI Hero board or FX-6300 mounted on a Asus Corsshair V Formula-Z board?
Intel gives you top notch performance but at a premium price, AMD gets good performance and no noticeable difference in gaming unless he can tell the difference between 120fps and 119fps, with a lower price. If there is no budget you also have to look at upgradability, overcloking (Intel's new solder instead of thermal paste is not the best solution for overclocking) and a lot of other stuff. My pick without knowing any specifics on the situation would be the i5 though.
Haswell for overclocking is more stable, but it does so at the expense of throttling the CPU a lot because it does get hotter and there is no way to turn off CPU throttling on any CPU so you either need to have a really nice cooling solution or you should turn down the OC a bit to get the best performance. Of course with Overclocking you want to go as high as SAFELY possible, but you should of course stay within the bounds of your hardware (ie: water cooling vs air cooling vs liquid nitrogen) and also keep in mind that the more it is overclocked the more power it will draw which will not just put a load on the PSU but also the electricity bill depending on where you are and how much power costs there. Intel still has the higher IPC so clock for clock Intel has the advantage but when you can take a 250 dollar intel cpu and a 200 dollar AMD cpu and have similar performance at stock speed with AMD leading in some tests (http://anandtech.com/bench/Product/697?vs=837) you really need to look at overclocking headroom as well. AMD's chips are great overclockers while Intel's new TIM really hurts the overclocking performance.