It depends on what you plan to use the pc for. Whether or not ddr4 may be beneficial, hyper threading, etc. A couple of things I thought I'd mention, if you decide the 6600k I'd opt for a somewhat better cpu cooler. The tx3 is a pretty low budget cooler and doesn't have the best performance (especially if you plan to overclock it). Another thing is the 32gb ssd drive, that's really small even as an os/programs drive much less anything else and is a budget ssd (which will also likely suffer in performance from its physically low capacity - the higher capacity ssd's perform better).
By the time you're ready to upgrade or looking at a worthy upgrade it will more than likely be on another motherboard platform/socket anyway. Many people opt to skip at least one or two generations to get any noticeable performance improvement, they don't generally upgrade every generation. People with sandy bridge saw no reason to upgrade to ivy bridge and by the time haswell showed up it was a new socket. Same for haswell/broadwell. Instead those with a sandy bridge based pc are just now considering 'possibly' upgrading to skylake, bypassing ivy bridge, haswell, haswell refresh/devil's canyon and broadwell entirely. It's more realistic to view the cpu and motherboard as a 'unit' that would get replaced together when it's time to upgrade rather than thinking you'll have a motherboard that will last you several upgrades.