It was always known that the eris was getting 2.0. Was stated before i even got my droid that it would get 2.0 and i got mine at release day. They stated because Sense UI was not up to date, they didn't want it to get 2.0 without it. So they waited until they could port 2.0 over to the eris before they upgraded to 2.0.
Your asking what does the droid get that eris 2.0 won't? Maybe be ahead of it by 1 updated? Droid users are going to 2.0.1, will the eris get this update, noone knows. But the 2.0.1 update is there to fix errors, and a few other things. I believe what you misinterpreted was that it DID come with 1.5 ot 1.6 or whatever version it came with. They just weren't getting 2.0 as soon as the droid was, because of sense ui. No one ever said they weren't getting it, and if there was a rep that told you that, then that rep was a moron.