What are your complete system specs? I'm just wondering if you might get more performance out of upgrading some other component.
If this were a case of buying one or the other, I would say to go with the 2700X, but it sounds like you are already running the i7-6700 in your system. The 2700X might technically be a little faster and have more cores available, but the differences in terms of performance in today's games should be quite minor, particularly since most current games don't really make heavy use of enough threads to benefit much from those extra cores. And unless you're pairing it with a high-end graphics card, you probably won't notice any significant improvement to frame rates, since today's games tend to be more graphics limited than they are CPU limited. I don't suspect an i7-6700 should have any trouble running today's games rather well.
It's also worth considering that AMD will be launching a new generation of CPUs built on the 7nm manufacturing process, probably around the spring or summer of this year, which will likely improve further upon the gaming performance of their current models. It doesn't sound like your existing processor should be struggling to run current games, so it might potentially be worth holding off a bit on upgrading from that processor.