If you have budget for 6700k go for it. Both CPU's have Turbo Boost but 6700k boost higher, when all 4 cores are in use 6700 will be at 3.7 Ghz while 6700k will be at 4.2 Ghz. I think that paying a bit extra is worth in this case, it will be much cheaper to upgrade mobo to z170 or z270 later on if you want to overclock than buying new 6700k and Z170/270 mobo for extra performance.Also if you swap motherboard you can reactive same windows you just need to select in options that you changed hardware.