What resolution/settings?
If you plan to overclock, the 7950 can get some pretty wicked clocks and with the additional memory and bandwidth, it's a better buy. Even without OCing, the 7950 > 7870.
Had you said the 7870XT, it might be a different story as at 1080p and below resolutions, the performance is eerily comparable.
However, between 7870 GHZ and 7950 Boost, I would definitely go 7950.