The "k" chip is an unlocked CPU, meaning it can be overclocked. The non "k" is a locked chip and cant be overclocked. Both chips will work on the B250 board, but the B250 chipset does not support overclocking. So if you get the 7600k and put it on the B250 board, you wont be able to use the overclocking feature. Basically, you will be paying for something you cant use.
If I were you, I would get the 7600 to pair with that board.
Additionally, you should only get either of these CPUs if they are a good prices as the i3 8100 has replaced these CPUs and they are $130 MSRP.