The 2D will be a bit "flatter" on the sides, but I don't think it'll be that much thinner overall, and I've found that gentle curves on a scale tend to hide it under clothing better than a transition to a straight edge like the flat sided scales.
The extra length on the 3.5 handle might come in handy for larger hands to get a 4-finger grip. I can do 4-fingers on my G3 handles and I wear a large men's glove with some room to spare. I could wear a medium but it'd be tight. So I guess I'm right on the edge of a M to L hand size.
I can say that the widest part of my palm, from the base of my thumb across to the outside of palm is 3.75". Measuring across the base of my four fingers that actually hold the knife handle, I get 3.2" Keep in mind that I use the choil when I hold my G3s. I could only get a 3-finger grip without the choil. If you want a 4-finger grip without the choil and you have a hand my size or larger, the 3.5 may be a necessity for you.
As far as 6:00 carry goes, I've tried that. The biggest problem I have with it is that it forces me to use just one specific hand to get to my knife. At 10:00, I can draw, use, and sheath the knife with either hand. I've been in situations before (work-related, not SD) that blocked my strong side hand from both my pocket and my belt. Having a FB available to either hand came in handy more than once. Besides, part of the 10:00 carry attraction to me is having the blade available when I'm sitting or driving. 6:00 doesn't work for me when I'm doing that.
Good luck with the search.