Comparison b/n D3 and L3+? ROI wise, seems obvious D3 is winner. Any reason to buy L3+?
Depends on how soon you can get either one. L3+ has already sold several batches, and D3 only a few. If you can get a D3 ordered in the next batch then it would be better than getting an L3+ in the next batch.
Only a few or just one batch?
Technically 2 batches sold on the D3 so far, but the second batch seems to have been tiny, or "leftovers" from the first batch.
As far as X11 difficulty goes - keep in mind that there ARE other miner companies building and shipping units NOT just Bitmain.
Baikal for sure when they have stuff TO sell and ship (right now they are "out of stock" on everything but "the Giant").
iBeLink is supposed to be shipping their new 10+ Ghash unit, and may have started shipping it a week or two back.