According to http://www.videocardbenchmark.net/high_end_gpus.html; 280 is benched at 1197, 650 is benched at 1837; so I would expect 650 to run faster than 280, despite its 512bit bus width.
If you are choosing which to buy, 650 would be my pick, 280 is far too old (at very least 5 years old, since it was released in 2008).