Apples and oranges and all that.
Both fantastic games. The Witcher is more of a slice and dice third person solo adventure, great story and very "adult". Direct combat via quick time events, and a handful of spells. Very straight forward with little fuss, no stats to worry about, no gear or weapons to dither over(at least none worth equipping). All together very RPG lite.
DA:O is party based. You can control up to 4 people and set fairly in depth sets of tactics for each person in the party. It's more of a traditional rpg in the sense you level up and put points into stats and numerous skill/talent trees. There is a hefty loot system, so plenty of shiny things to switch around for you and the party. The combat is more tactical, less hack/slash, choosing the right talent at the right time via a bar, like WoW for want of a better example(it's one most people have played).
Both very different, but excellent in their own right. At the end of the day I would take DA over the Witcher just because I felt the Witcher had no replay value.