vampires you can play around with a lot but there has already been quite a lot of popular vampire books. I would suggest demons if you had to chose between those two. Dont forget there are loads of possiibilites for monsters so you can just make one up or look up myths online. Im writing a story as well at the moment and i have got this really cool book with all sorts of legends in it so all i do is pick a few then change them a bit. Also i chose a main chareceter. You could do that online or you can look out for a good myth book.Anyway i have rambled quite a lot already so all I can say is…
Well I prefer vampires (darren shan ones) however, if you are writing it for a book, choose demons. There have been a lot of vampire books published and not many demon ones. If you can raise awareness for demon lovers that you have wrote a demon themed book, then more people would read. It’s all about marketing. Write your book online, Wattpad is a great place, I write my books on there, and then advertise it wherever cheap/free and possible. You could also make forum posts to advertise your book on here. On Wattpad you can design a front cover and a blurb instead of just the story.
Demons. There’s more room for creativity with demons. There’s a general outline for vampires. They suck blood, can’t come out at night, look fairly human, etc. But there’s no outline for demons other than completely evil, so you can do more with them.
I voted Vampires, but when I think about it, Demons might be more original(ish) because there are so many books about Vampires, Demons may also seem more interesting to the people who’ve read about Vampires already, or who don’t like the other stories and no longer bother. Plus, like Serene said, Vampires have all their information and facts created already, but you have lots of room to expiriment and create with Demons.