When they appear in DLC, I see them as the devs having fun with the game, and letting the fans into that fun, too. I strongly doubt that when RDR2 comes out, the zombies will be considered story canon.
One thing I haven't seen posted here is that there is an exclusive Gang Hideout and an additional outfit for the PS3 version. These aren't the same as any pre-order bonus, so the only way (for now) to get them is to have the PS3 version.
Personally, even though I own both consoles (and a...