What are the differences between the SR 71 variants?
Like every military aircraft built over the years, the aircraft evolve.  So to address the aircraft's history and all variants, let's refer to it as the Blackbird.  

The Blackbird started out as the A-12 procured for the CIA. Fifteen of these were built with one being a 2-seat trainer. Two of the 15 were modified into M-21 aircraft prior to delivery.  The M-21 was designed to launch the D-21 drones. The SR-71 at the Intrepid museum (60-6925) in New York was built as an A-12.  One of the M-21's is at the Museum of Flight in Seattle with a D-21B drone.

Three high-speed interceptor prototypes were built and designated as the YF-12A and were capable of carrying 3 air-to-air missiles.  Two crashed and the third is at the National Museum of the AF.  

The interceptor was cancelled and the RS-71 program, renamed SR-71 after LBJ's misstatement at the press conference, went forward for the AF.  Twenty-nine SR-71A's were built along with 2 SR-71B trainers.  When one of the trainers was lost in 1968, the surviving aft part of one of the crashed YF-12A's was mated to the front part the SR-71 static test aircraft to create the sole SR-71C.

There's a huge Blackbird community online.  Detail specs and history are available at blackbirds.net and at SR-71 Blackbirds.

The question of which is superior is an opinion, but one can assume that the SR-71's were "better" than the A-12's as they came later building on what was learned from both the A-12 and YF-12A.