5770. It is better price/performance at $150, has dx11, better crossfirex scaling, lower tdp, lower heat, better overclocking (if yours has vGPU control), openCL, a true onboard sound controller, better video playback/decoding, openGL 4, is more reliable, and is more futureproof (in case you wanna Xfire later on...)
the 4890 only has higher framerates, and not by a lot on newer drivers.