GTX 770 = $450
R9 270X = $249
R9 280X = $440
R9 280 = $360
GTX 770 = R9 280X
GTX 760 = R9 270**
**There is absolutely no point in buying the X version since its the same as R9 270 with a faster clock. Buy an R9 270 from Asus or MSI you can overclock at beyond the manufacturer stated OC. My Asus overclocks at 1175mhz core with stock at 975mhz.
If you are on a budget R9 270 is better choice since its cheaper and it has Mantle and only 2-3% performance difference to a GTX 760. If you want to spend over $100 for a just a couple of % performance then go right ahead to nvidia, but it makes no logical sense to overspend on graphics card when 4 years later it will become outdated.