5000 is the oldest and can either be a swather or combine header with an adapter kit.
5100 is a combine header only, i think was dropped from production when AGCO came out with the 9250 Dyanflex about 2010 or 2011 if i remember right?
5200, same as a 5100 but is a swather head only. Came out in about 03, 04 i think and is still built today. Was told it will be replaced in 2015.
5100 and 5200, while basically the same head were not interchangeable between a swather and combine like the 5000 was. Had something to do with how the adapter sat in the 5100. I personally have more experience with the swather end of things. All are good solid heads imo. The 5200 is a step up over a 5000, but is not a poor head by any means. Imo the are as good or better in some respects to a MacDon, especially when is comes to swath formation and flotation.