In your original question you make it look like a choice between the GTX 960 or the R9 280X. Of those two the R9 280X is the clear choice. A quick Google search of 'GTX 960 vs R9 280X' will give you plenty of results with many forums recommending the 280X.
As it is, Nvidia own the top bracket for GPUs (Titan X, 980TI, 980, 970), but if you aren't going for one of those cards then the price/performance ratio of AMD's cards is what really shines.
My honest advice on this topic: wait a week or two to find out what AMD's latest line of cards bring. Their new 300 series could wipe the floor with Nvidia. And even if they don't, their 200 series cards will come tumbling in price anyway and you'll find yourself a nice deal. If you rush into buying a GTX 960 on the Nvidia hype right now you might as well just tear up half the money you're spending.
EDIT: Here's the pricing of the new 300 series: http://wccftech.com/amd-radeon-300-series-pricing-confirmed-aggressive/