SoM : Platinum in 30 hours, pretty fun, very similar to Assassin's Creed's gameplay, but in the Middle-Earth's environnement / lore. Not much replayability value.
DA : A nightmare difficulty playthrough length : between 80-130 hours, amazing replayability value, tons of lore / letters / books / dialogues.
Having played both thoroughly, I would definitely recommend DA, even though a playthrough of SoM is a great ton of fun. It just never gets boring to play DA, and you can spend easily 200+ hours on it over the course of multiple characters / difficulties.