Some of them will have different overclocking capabilities. If you're just going to plug it in and play, pick one and run with it. If you want to overclock it, you need to do some research on the VRM type, the memory type that it's using, the power limits set in the bios, voltage restrictions, how many 6 / 8 pin connectors it has...all of those things will come into play.
For instance, the ASUS Strix 970 has 1 x 8 pin connector and is voltage locked at the VRM to 1.212v. Same with many of the cheaper Galax cards, and the MSI 970LE.
Some cards are cheap for a reason.....
I don't know of any manufacturer that voids the warranty with overclocking.....