Hello, the difference is micro architecture. The 4th generation intels are based off of the Haswell and for the high end gaming cpus, the Haswell-E micro architecture. The 5th generation intels are only based off of the Broadwell micro architecture. The Broadwell-E or 5th generation high end gaming cpus are not out yet. The difference between Haswell and Broadwell is not much. Intel follows a cycle when releasing micro architectures. It is a "Tock, Tick" cycle. When a brand new and different micro architecture comes out it is the "tock" The next release is the "tick" in which they take the same micro architecture as the "tock" but shrink it and make it more efficient in terns of heat, power, and performance. The Haswell or 4th generation cpus are the "tock" and the Broadwell or 5th generation cpus are the "tick"
To sum it up, Broadwell is only just coming out. If you want performance now, go with 4th generation because 5th generation is only in it's early stages and they are not much powerful than the mid or late 4th gens. But if you wait to lets say end of summer and autumn then the more powerful Broadwell cpus will come out, then go with 5th generation. But either way, it is your choice. Happy choosing!