Which CPU is better for playing multiplayer on Minecraft?

Its mine craft it doesn't really matter

Minecraft is not really CPU or GPU intensive. Either one of them will be more than adequate.

But that being said, if you want to know how the processors compare to each other in general, then the i5 is better at single-threaded workloads, while the FX is better at multithreaded workloads. CPUBoss measures single-threaded performance, and they rate the i5 at 8.6 and the FX at 8.1, which is large enough to be slightly noticeable a difference in single-threaded workloads. PassMark measures multi-threaded performance, and they rate the FX at 9022 vs the i5 at 7670, which is also a noticeable difference.

I really would go with the amd fx 8350 8 core processor

Better off with the i5 4690 Haswell as you're relying on better cores and not more cores for minecraft