A card can only be 'better' - having a better benchmark eg Cinebench, Metro 2033 with specified detail. One card might not always be better in all benchmark testing. Eg change detail - MXAA, FXAA, resolution (1080, 4k) or game. Your CPU can also affect the benchmark results. And there are errors in results. If a card is at least 10% better than another in one test it will probably be better in most tests.
But also better can mean features - GSync, Mantle, SLI, XFire, etc and overclockability, temperatures, noise, power consumption.
So 'better' isn't as straight forward as it first might be thought.