With the release of the 10 series, I was wondering if getting a used gtx 980 for $250 to pair with my own 980 is a smart decision for gaming. Or would it be better to sell my 980 for a 1060? I've a 1440p display BTW.
Better off selling the GTX 980 and getting a GTX 1070/80. GTX 980 is pretty much equal to the GTX 1060, and a single powerful GPU is always better than 2 less powerful ones.
If you have a 980 you shouldn't sell that one for a 1060. A 980 is better than a 1060. If you will sell your card, you can get a 1070 which is even better than a 980ti.
If you don't want to buy a 1070 you should get a 980 in SLI.