I used to have knowledge on graphics cards, but that was last gen (or two generations ago?)
I don't know anything about the graphics cards you're talking about, but I do know how you can find some useful info on them.
1) If you're looking to build a new gaming computer, it would be a great start to ask the forum "Build me a gaming computer that can..." and list your needs. Many members will give you cheap and effective builds. I think there is a forum specifically dedicated to this. Your needs will probably be to run a certain program or play a certain game. List those.
2) If you're looking to just buy a GPU, the first step is to find out what kind GPUs are compatible with your motherboard. Once you know, then look up the programs or games you want to run and check out how different GPUs perform while running. Or you can use the newest games on the market as a bench mark. Googling "Far Cry 4 GPU benchmark comparison" should give you graphs of different GPUs and how they compare in performance.
If you have any more questions just ask