Hmm... Well I only have a 1/4 size doll so I don't have any other experience to base my preferences on.
Just, the MSD size always seemed to be the "natural" choice for me. When I first discovered BJDs and imagined myself with one, the doll was always MSD sized (I have no idea why, and it wasn't because I wasn't aware of the size differences since the first time I saw them was at a doll cafe with all the different sizes there).
If I were to buy another doll to join my slowly growing family, I think I'd like to have a 1/3. Partly because I do like the 'mature' look SD sculpts tend to have compared to other sizes, but also because they're easier to shop for.