There's not much at all in it between the two coolers. The U9S seems a lot more popular in M1 builds but both perform to within a couple of degrees - and which comes out on top seems to vary depending on which reviews you read (generally it's the U9S but not always).
The U9S is a bit heavier so you could argue that it has metal and thus more fin surface area, which should lead to better cooling, but it's marginal in my experience. The D9L is very slightly shorter (110mm vs 125mm) which may be important if you're trying to fit anything on the side bracket above it - there's no room there if you're using the U9S.
Personally, I have a U9S in my M1 and I'm very happy with it, but if I'd not been able to get hold of one for whatever reason then I'd have happily gone with the D9L (or if there had been a significant price difference, which there wasn't).