They're practically the same card (same core chip), the R9 270X is just clocked a little higher. Price is the only realistic way to separate them. Or you could look at the stock R9 270 and try to overclock it to the 'X' speed.
:P or he could take the 270x and overclocked it to a close-to 280 speed
^ Won't make up for the R9 280s better chip, though.
Still.......unless u are on an budget OP i dont see why u would take the r9 270 instead of the X version, i may be wrong tho but thats my opinion