What makes using OBD2 system, so much better than OBD1

Reading about using factory efi on an engine swap. OBD1/ OBD2 . What makes OBD2 such a better thing to do ? Also if going to OBD2, would using an A500re/A518re be the better choice ? Are the RE transmissions reliable for a DD cruiser? Does the engine ECM also control the A550re/A518re or does it use a separate control computer ?
Asking these questions so that I know what to get as a donor vehicle,