Well you're in a tight spot because your CPU and GPU compliment each other well as far as price/performance. If you upgrade one you would likely want to upgrade the other.
Forgetting about that though, I would say right now a CPU upgrade is better than a GPU upgrade for a couple of reasons.
1.) Coffee Lake just released and 6-cores and quad-cores are [somewhat] available for a good price from Intel, and even AMD has their good prices.
2.) GPUs are on their second year of the same generation. You wouldn't get as much time out of a GPU right now as you would a CPU upgrade.
If you do upgrade your CPU, it should either be a 7th-gen i7, 8th-gen Intel, or Ryzen 5 1600 or higher. But I think you have more time with your current setup before you actually need to upgrade. But I can also understand wanting to upgrade when you don't necessarily need to.