Here are the main types of configuration:
Key:
Year available, Model name, Cooling, Engine Size
1972 - 1977, Standard, Aircooled 594cc
1977 - 1984, Standard, Aircooled 652cc
1984 - 1994, FL, Aircooled 652cc
1987 - 1992, BIS, Watercooled 704cc
1994 - 1997, EL, Aircooled 652cc
1997 - 2000, ELX, Aircooled 652cc
The stopped selling the 126 in the UK after 1992, so you don't get UK versions of the EL and ELX.
The "best" configuration is up for debate!
The ideal person to answer and make comparisons, is someone who has owned both an aircooled and a BIS.