No comparison. The 270x is somewhat better. Some will say, well you can overclock the 270 up around 270x performance, which is true, but of course you can also overclock the 270x beyond what the 270 can handle. So at stock speeds it whips it and when overclocked it still whips it, but probably not by that much.
It depends on what brand and model you get and what the base clock comes set at. For most models, there's about an 11-17% performance difference between the two. Since you can get a 270x for about six bucks more than the 270, I'd say it's a better option.