They're very, very similar. I think the main difference is that the D16 has a glossy finish whereas the D15 is flat (satin). I've owned both a D15 and D17 from the mid 80's... fabulous all mahogany bodied guitars! I think there were also substantially less D16's made vs. D15's.
My '87 D-15 originally had a high-gloss finish, but through the miracle of fine-grained sandpaper and liberal amounts of elbow grease, it now sports a satin finish. It's a personal preference thing, for sure, and I didn't care for the 'screaming' gloss.
It's still a nice little mahogany box, made to be played.