im not sure it really matters.
R350 core : slower shader perfomance but seems to overclock higher
R360 core : faster shaderperformance but doesnt seem to overclock as high as the R350
THis is data i've seen from multiple hard ware forums, all the threads/forums dealin with the pro->xt mod, and personal experience. I had a 128 r350 pro oc to 445gpu, and my current 256mb pro ->xt only goes up to about 425gpu. both had aftermarket cooling and the xt had the better one.
But i might be wrong...