I have a gtx1060 6gb with a i5-7600k @ 4.7ghz, I want to upgrade my gpu but I don't know which card will be better for 2k gaming. I heard mix things about the 1070ti being better than the 2060 and vice versa so I dont know which would be the better fit. budget is around 350$
RTX 2060 SUPER is close to a GTX 1080 also has faster VRAM on the both versions but the non super version has less VRAM (6GB's). Me personally, RTX is way to overpriced and not worth it with such a small selection of games actually utilizing it. Go with a 1070 Ti and later down the line when the prices drop a little get an RTX 2060 SUPER or 2070.
If the price for the GTX 1070ti is cheap then go for it, RTX is not yet fully utilized on all games and if you want it to be used it will decrease performance as well even. If you go for RTX 2060 you will have to dial down some settings on the game. so GTX 1070ti can do the same as previous NVidia statement that starting from 1060 6gb to 1080 Ti can use the RTX feature but with the performance not that good,
however it a second hand try to ask your buyer if he can give your a personal warranty and ask details what's the reason selling it and did he use it for mining or so at least you have idea how the card currently holds up.
Or even try to look for 1080 Ti there might be cheaper that you can get who knows.