Hi Sig. I noticed no one has replied to this thread yet and it has been sitting here a couple of days, so I thought I'd throw my opinion out there. Let me preface it by saying I do not have personal experience with either of those uke models in koa in particular. That said, I do have experience with LoPrinzi and also around 20 years experience in stringed instruments of anywhere from low end to high end. So that's where my opinion is coming from.
When you get to the quality, professional builders like LoPrinzi the highest end models are higher priced mainly due to cosmetics. It is entirely possible that you may like the sound of the Model B more than then Model D. It's also possible that the difference in sound of them to your ears is so subtle it doesn't matter. Keep in mind every individual instrument can vary slightly too. So maybe there's some D's that you will like better than the B's and vice versa.
Sound is entirely subjective and impossible to describe with words. Ideally you'd be able to get your hands on some of these to try them for yourself. I realize that can be difficult with LoPrinzi though since there aren't a lot of them made compared to some other brands. As far as I know, it's mostly a one woman shop (Donna LoPrinzi) with the uke building.
If the higher end uke with the upgraded cosmetics inspires you to play more, then maybe that's the one to go for. That's a personal preference thing too though. I do realize that sometimes if we like the look of an instrument we are more inspired to pick it up and play it.
I got to play on a model B cherry tenor from LoPrinzi for a while and it had great build quality, playability, and sound. The LoPrinzi that stands out the most to me though is a model A solid mahogany soprano I had for a while. As far as I know, the Model A is the least expensive and adorned uke that LoPrinzi makes. Even so, this uke had gorgeous grain in the mahogany wood. The sound was outstanding, and the build quality was top notch. It didn't have any added bling, which I actually prefer. That was one of the finest musical instruments I have ever owned and played. I ended up selling it because I think I went back to tenor for a while, or maybe was focusing more on guitar at the time. I do have some regrets about selling it sometimes. I think I paid around $400 for it, and I found it comparable to instruments twice the price and then some in terms of quality.
I just don't think you can go wrong with a LoPrinzi in terms of quality. I think it's one of the better values out there too. If you get one, I hope you report back here with your experience, pictures, etc.