Either one of those upgrade choices will leave you with a severe bottleneck. That said, if you're itching to buy now and can only afford one, I'd get the GPU first. Hopefully you'll have enough for the CPU by the time AMD's 3000 series Ryzens release, which should drive down the 8700k price. If you plan to do lots of streaming though, one of the better Ryzen 3000s would actually be a better choice. Ryzens take at least 3200 RAM speed to run well though.