But there both 3.4 and amd is cheaper
Those two CPUs aren't even in the same league. There is a very good reason the 750k is a lot cheaper. It is not anywhere near as good as the i5. It does have a lot of value and is great for a very low budget build but that is all.
In a pure performance comparison the i5 wins hands down in every respect.
If you are looking to a lower cost alternative to an i5 you'd have to look at the 8320. The performance of the 8320 isn't as good as an i5 in gaming, but it is much much closer than the 750k. And in things like video editing and video encoding (if you do that kind of stuff) it performs a bit better than the i5.
If getting the 8320 would get you a better graphics card then I'd recommend going with it over the i5, but if you wouldn't be able to get a better graphics card, then get an i5.
Long story short, unless you are on a very very tight budget, don't get the 750k.