(Forgot the loggin for my alt, so frick it, I'm using my main!)
I can't say much about Duke's butt since I don't own one myself, but I can talk from experience about Nix!
Nix is unique in that she's both larger and heavier compared to the other masturbators available. Because of her size and weight, Nix can be pretty hard to use one-handed (which might be a deal-breaker), however the weight makes her the perfect toy to hump.
My favourite position to enjoy Nix in is with her on her back ontop of a table at crotch-height. She doesn't move while in use, and it's so much more enjoyable to me being able to thrust into a toy without using hands. The tail can get in the way sometimes when used on her front, however I absolutely love the sensation of the tail rubbing against my balls.
The cloaca/main hole feels great and is nicely textured. The tail is curved smoothly to guide you into her entrance, and once you start going she pulls you further into her depths. If you want to switch things up you can use the other end for a completely different sensation!
My only real gripe with Nix is that she's a little on the loose end for me, and is difficult to use as a traditional masturbator. She can also be very noisy when lubed up, so if you're living with others I'd recommend plugging the other end. That said however she's my favourite toy Bad Dragon has released yet. It feels like you're with an actual dragon!
TLDR; Get Nix if you want a humpable dragon booty