The R9 270x is a rebranded Hd 7870 ghz edition. So the R9 is a hair faster then the Hd 7870. Like 2 fps difference at the most. Although you can overclock the 7870 to be just as fast.
R9 270x and HD 7870 are not the same gpus. R9 270x is more future proof as it support new AMD "mantle" technology and also "eyefinity" technology. For now mantle in games is used just in Battlefield 4 (allows to play on ultra settings), whereas older eyefinity cards with the same specs as R9 270x costs a lot of more.
The 270X and 7870 ARE the same. Mantle is an API for use on all GCN architectures, eyefinity has been around forever and you can Crossfire a 270X with a 7870 which means the chips are the same