I agree, it’s wroth a gamble if you knew the seller personally, like a good friend selling hw for upgrades, if it’s ebay or craglustlist no, There are lots of scammers, if you can verify that the card is indeed a 980ti, and the card is not a mining card. 980ti give you a much better performance boost over the 1060.
Also it has to do with the fps and resolution expectation of your setup. 1080p should be fine for both, In cases of 1440p gaming, the 980ti has a slight edge over.