From what I was told when we were looking like OP is, is the exact same thing. In '05 when we bought our Silverback, that line was slightly different in terms of floor plans and extra goodies on the Creek line. Then the Silverback line went w/ the cathedral ceilings to really differentiate themselves from the Creek line. Was told last year however that Silverback will be dropping the cathedral ceilings. Big Country the same way. Different floorplans yet same assembly line. In comparing similar floorplans, you really have to look for differences as they maybe subtle. Look at the bedroom fireplace offerings between BC and LM and how they are layed out. In fact, I think in the BC, the '19's don't have the stone facade anymore. The fireplace in the LR on BC now have a "marble" mantle trim to help dress it up.
Guess you could look at it in an entirely different way like in terms of autos from bottom of food chain to top dog.
Ford, Mercury, Lincoln
Chevy, Buick, Cadillac
Chevy, GMC
Big Country, Big Horn, Landmark
Silverback, Creek (Hathaway now), Champaign
All are similar yet the edge will always go to next trim level/brand up. In the case of Chevy/GMC, years ago there was a very large difference between the two. Today it's down to the GMC having extras not avail to Chevy (at least it used to be that way last I knew)