FWIW: depending on type, a headset and bottom bracket could easily weigh around 400g combined - which appears to be roughly the weight difference between the two frames in question. If the Mercia's quoted weight is sans headset and bottom bracket, then the frames likely weigh pretty much the same.

Either should make a fine vintage bike.

If possible, I'd look carefully at both frames in person. Absent a compelling reason to the contrary (e.g., dramatically different geometries, absolutely unacceptable color, etc . . . ), I'd then probably select the frame that appeared to be in the best shape.

Best of luck.