Although they don't publish exact numbers, Schwalbe puts the Marathon and Marathon Plus in the same category regarding rolling resistance. I'm sure my Pluses will have more rolling resistance than my standard Marathons if for no other reason than the Pluses I bought are narrower. But how much more resistance? I guess I'll find out. Whatever it is, I'm certain it's still going to be lower than the knobbies I rode (and even toured on) for years.

Schwalbe has an interesting article on rolling resistance here.

Edit: To the OP - I have been very happy with the standard Marathons, and I have always run them with liners, but no inflatable tire is flat proof. I would not hesitate to buy them again. I only bought the Pluses because I got a really good deal on them. If they don't meet my expectations, next time I will go back to the standard Marathons.