While they are nice, the Wed to Darkness horns are a little too thin looking at the points. They were okay for wearing for a while but I like my black ram horns better because they do a nice job of hiding the base ears. As for tails, the only one for me is the Demon Vassal tail. I love the way the end of the tail is a rounded point and how the shading is a nice complement to the shading on the horns I put on my avatar and how the shading on the bow goes with the shading on my ears and butt wings as well. Plus, the little bow adds a nice little feminine touch to the tail. It doesn't matter that the tail and horns stand out against a lot of the outfits I put her in because of how it's placed on the avatar that balances things out nicely. I like that it adds another dimension to her overall character.
I'm not sure how many tails/horns they've released but we could always use more (can't go wrong with variety, Gaia! Just sayin'...).
Hmm... I guess my favorite horns are still Horns of the Demon, you'd have to pry them from my cold undead fingers to get them away from me. Lol. My favorite tail would have to be the Queue Demonique, I like the curve and the width (not too thick or too thin) and it goes well with a lot of the different outfits I make.
I've actually given up my quest for a D-Tail, which is fine by me because now it looks a little too brown for my taste.
As for Demon manifest, the horns are a bit fat but I guess they didn't want them to look too much like HotD. I absolutely love the 'pierced tail' pose though. I'm not ecstatic about 'petite horns', but I know I'll end up using them if I ever make a pink/black avi. The 'coy tail' on the other hand, I will probably never use, as it passes in front of the avatar.
And wings, GOD(?) YES! More wings, PLEASE!
((The ones you can make through alchemy look amazing, but are just way too expensive to buy/make.))