I really liked DKS; but I played it well after it was cleaned up. DD was ok; I definitely prefered DKS.
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My issues with D:OS is not with the combat length but I think the game (story) is not as well fleshed out as either DD or DKS (or maybe it is but not presented as well). If I had to 'rank' the games it would be somethign like:

7+ for DD (won't go into detail here; and I know many would rank it higher)
9+ for DKS (wished it was twice as long and did not esp like the very end where you have aerial combat)
8.5 for D:OS (not finished yet my opinion might change; also I'm sure patches will improve it and add more content so 8.5 is sort of a bottom score at this point; the problem is that there are a few specific irritation points that really irritate me - such as very slow movement - walking across the city multiple times is a slow experience and portals not being labelled; as well as discovering because i lost a random RPS event two quests are now undoable (headless and entertainer - more upset that i can't find a way to do headless).