That GTX 980 Ti is the most powerful of the two, but it is used. So I generally don't trust used computer parts. If you are just wanting 1080p, and don't play on upgrading your monitor, the RX 480, or even the RX 470 will do. Save yourself some money. Of course the GTX 1060 is good too, but if 1080p is your max, then just go with teh 470/480. If you DO plan on upgrading your monitor the 980 Ti will benefit you, but you may not know if it is damaged iniside.
Again, that's just me. I trust you investigate what you need to.