@scrapjockey,
There's no difference functionally when it comes to operating your command control switches.
There's a bigger difference from a user interface point of view. You basically have 4 choices:
1.) 6-12969, Classic TMCC Command Set (Cab1/Base1); does TMCC Only
2a.) 6-37155, CAB-1L Remote Controller; does TMCC and most of Legacy
2b.) Base-1L, Part of 6-37147, Cab-1L/Base-1L Command Set; does TMCC and most of Legacy
3.) 6-14295, Legacy Command Set (#990), Cab2/Base2; does TMCC and all of Legacy
4.) 2208010, Base-3, No Cab3 will be produced; you'll need to use use an existing SmartPhone, a Cab2, or a Cab1L with this; does TMCC and all of Legacy, but depends specifically on which Cab you use with it.
Be aware that the Cab1/Base1, Cab2/Base2 and Base 1L are no longer manufactured, only Cab1L. Also, Base3 is forthcoming and not yet in production.
Mike