The X1950 XT because it is much, much faster.
Why? Better architecture and faster clockspeeds. The 7950 GT has 24 pixel shaders, the X1950 has 48 pixel shaders... the 7950 GT has 1400 mhz memory, the X1950 XT has 1800 Mhz memory...
Also better image quality with the X1950 XT, but that's a side issue. In the final analysis, the X1950 XT will provide a much better gaming experience than a 7950 GT.