A12 was the same model but built of lighter materials and much faster.
It wasn't that much faster, I think it was 0.1 mach faster, anyway the SR-71 design was a much more versatile and a 2 seater and they really needed two pilots to get the missions done that they wanted efficiently, even though one wasn't really doing a pilot stuff. Anyway the SR-71 could carry more camera's and spy equipment then the A-12.
The A-12 was designed as more as an interceptor aircraft, or at least that is way it was sold to congress for funding.