I really hope it's more sandbox. They have such great RP/replayability.
I played Dragon Age (the latest one) and Divinity II.. I think it was, and while they were pretty decent games, there's no way I'd play them again.. it's all pretty much the same on the first play through. A Sandbox game can be played over and over and over and doesn't get old for a long time, and are definitely more memorable. See.. I barley remember the name of those other games... lol
Sandbox games are like GTAIV, Red Dead Redemption.
Open-Ended games are games where you can make different decisions that will alter the game, like Fallout 3 and Oblivion (which can also be considered sandbox, because of the open world to explore), Dragon Age and even Mass Effect. Most RPGs are open-ended, not all of them are sandboxes.
As for RAGE, I think it will be as open as Borderlands. A kinda of a large world divided into various individual maps.