AMD are probably better. AMD cards are typically better at pushing higher resolutions, not by a huge amount though. 8GB 290x is going to perform better at 1440p than something like a 4GB GTX 970/980, for the most part. It's worth waiting for the new cards and check out some benchmarks, make your decision after they've been released. 780Ti will be fine to hold on to for now for the next 4-6 weeks or so.