I've owned a D-45 but prefer the 28 I have now. I do believe the 40 series sound different (I've had a D-42 as well) to the 28s. I think the scalloped 28s are boomier while the 40s are more balanced. Perhaps there is more to abalone than folk think, or perhaps not.
I think the D-45 is a bit of a rip off however. Unless you find one that really really blows you away I think they vary just as much as your average 28, but they do sound different (in my experience anyway).