Truth be told, your CPU could easily handle even Nvidia GT750Ti without seriously bottlenecking it. If you can find one used for cheap, go for it.
Between the two cards you mentioned, R7 is slightly faster (by approx. 20%). Although both cards are at least twice as fast as your current one, they are both utter crap for gaming, except if you are ok with playing at 720p in low details and mostly e-sports games with sub-60 fps.