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.