He is right, a 270x tends to be closer to a 7950 since most come factory overclocked. I would go for the 7870 and try to overclock if you are comfortable with it.
To give you an idea
Cheap 7870= Pitcarin GPU running at 1000Mhz
Cheap 270x= Pitcarin GPU running 1050
Good 270x= Pitcarin@1150-1200
Cheap 270 non-x= Pitcarin at something like 950-1000Mhz