The Digital Design version is clearly better, especially in the field of ground textures, and it also has plenty of landmarks around the airport that the World Update MS version doesn’t have.
This is not to say that the MS version is bad, but if you already have the DD version, keep it without looking back and remove the Microsoft One.
To respond to question 3, but you may already have guessed, you’re actually losing custom POIs if you use the MS version instead of the DD version. The MS version doesn’t have custom POIs outside the airport. the DD version indeed does.
I’d be iffy advising someone to buy the DD version when they have the MS version for free, but since you already have the DD version, you can be confident in your purchase and keep it installed. It’s great and the MS version is only “good enough.”