Modern graphics card should not saturate a 2.0 x 16 bandwidth. Go with what ever card works best for you. There are several reviews out there which show you really only start to lose a little performance at around 2.0 X 4 and below.
edit -- the 270x is just a rebadged 7870, that is why the performance is so close, I believe they just pumped up the clock frequency on it. I would just grab the cheaper of the two and over clock slightly.