I think gtx 960 will be a much better option because it has got new technologies and it's performance is much better than the r9 270x and it also consumes less power than the r9 270x it's a power hungary gpu you have to get a good psu of 600 watts that will cost u around 4.5k for a decent psu while the gtx 960 requires only 400 watt power consumption get the gtx 960 you will never regret your buy
And if you love ubisoft games than go for nvidia otherwise if you already have a 600 watts psu and money is an issue than go for the r9 270x that's also a wise choice