4590 vs 6500

Solution
i would use the 4590 because Skylake only has a very little performance gain from Haswell. and you have to but more expensive DDR4 RAM
The decision breaks down to newer platform with better IO and slightly better performance (Skylake, 1151, i5 6500) vs lower costs with very similar performance (Haswell, 1150, i5 4590). If you have a budget concern I would definitely go with Haswell.