The Lunar Duo is a big tent. The SS2 seems like it's a nice size too, but it's not going to quite as a big as the cavernous LD. SMD claims a 54" x 90" floor for the LD, while the SS2 is claimed at 52" x 86". The arch poles used in the LD likely provide as least as good of headroom as the offset poles design of the SS2, although I'm speculating here.
My point is simply that you aren't going to gain space moving to the SS2 and you'll likely lose a little bit. However, since the SS2 is a double wall, you can freely utilize all of that space. With the partially single wall LD, you need it to be bigger so you have room to stay away from the potentially wet ceiling. You could call living space a slight edge for the LD, or a best a draw I'd guess.
IMO, the primarily reason to switch to the SS2 would be because you want a full double wall tent and you can make the switch with virtually no weight penalty. Would you rather not have to deal with the condensation on the underside of the LD's fly (ie. do you sometimes contact this with your feet, head or elbows)? Or does it not bother you at all? If your wife isn't feeling the potentially wet ceiling, then the switch the SS2 could be a wise move.
One other difference is that the LD has mesh pockets in the inner, which is handy. The SS2 doesn't.