If you are happy with performance now I say wait. If you try to play games in the future that are more demanding and your 280x can no longer keep performance up to the level you want that is the time to upgrade. Who knows.... there might even be new, more powerful GPUs available from Nvidia and AMD by the time that day comes.
trying to buy computer components today that will be good for the future is a loosing battle. There are always more demanding games coming out and there are always more demanding components coming out to handle them. Your future proof card you buy to replace your 280x today might get destroyed by a card in the same price range 6 months from now. That's just the way technology works.