Assuming your system is running just fine, and assuming you have a budget (since you are asking to get one or the other): Neither. Put the money in a savings account and see what is out next year.
For your GFX card, it is plenty good enough to run everything out right now, so no need to upgrade. GFX cards are too expensive to do one-step upgrades on the hierarchy chart, unless money is no object. Wait until you can jump up 2+ tiers.
For your CPU, you could certainly use an upgrade, but due to the just-started new war between Intel and AMD, there are going to be some amazing new CPUs coming out in the next year or two. Anything out right now is a Ryzen, which underperformed in gaming, or is an intel stuck-in-the-mud chip that is the product of the last several years lack of innovation due to no competition. In the next year, EITHER 1) games will be optimized for Ryzen and they will perform fantastically, OR 2) Intel will come out with its first real innovation in a long long while due to the Ryzen competition. Or both will happen. In either case, the current crop of 2017 processors will look very boring by then.
So my suggestion is to wait for now. That's what I've come up with for my own plans. Just for the record, that's not my usual approach. There's ALWAYS new tech coming out, so you can get into a game of waiting forever for the 'next best thing' because there is always something better on the horizon... I hate that trap. But right now we really are on the cusp of big improvements, IMO, due to AMD's surprise attack. This time, unlike usual, it may actually be worth waiting for the next generation.
If you are unable to control yourself and must upgrade something, the CPU.
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