For ~$200 the 280 is the best option. It's faster than the 270x, 4gb of vram on a 270x is useless(and overpriced) unless you are using 2 in crossfire, a single 270x isn't going to be able to make use of 4gb.
The 280 is a faster card, it has more cores. It can play most games at 1080p with settings maxed out or pretty close to it. It's actually the same card I have with a different name. The 270x is much slower.