The R9 280 is a rebranding of the HD 7950.
So in a chart: R9 280 < R9 270X < GTX 760. That said, the HD 7950 is an awesome overclocking GPU. I have not tested the R9 280 so I don't know how well its overclocking potential is, but if it's similar to the HD 7950, it can be a really good buy. But if you're not going to overclock it, then the hierarchy remains as is.