29": Big wheel, rolls over objects better and carries speed better. This is why it is heavily seen in XC racing. But has more rotational mass and allows for more torque on the wheel, thus 'easier' to bend. The larger wheel also means it is more difficult for manufacturers to make long travel 29'ers, it's why you don't see many of them in long travel versions. Further the larger wheel makes the bike less manuverable (depending on geometry anyway) as a general rule. This is why few ride 29'ers in Enduro racing/Super D.
650b is 27.5" with a stupid stupid name.