Hi Gary, welcome to the forums.
The BIS is a later model of the 126, its got a boot as the engine is underneath the boot floor and is water cooled, it has a 704cc engine rather than the aircooled 652 and 594cc engines.
I only have the aircooled ones so I'm not sure about any more differences but Pete 126 has a Bis and I'm sure he will know lots more than I do. There are also lots of other people who would be happy to help!
Good luck with your search, a little more on what you are looking for might help, condition, year, aircooled/watercooled etc etc..
Chris