For Neverwhere, the BBC radio show came first, I think.

Someone who really likes hard sci-fi would tend to like Tim Powers more- he seems to take an idea and write a book around it- what if magical time travel was possible, what if ghosts existed, what if the Tarot was a way to contact certain segments of humanity's unconscious collective.

Neil Gaiman, on the other hand, I think is trying to bring legends into the modern age- we're sitting around the campfire, and listening to grandfather spin tales, that are based on the tales his grandfather told, etc, etc. Odin and Loki are con men trying to gain power in a fight between the forces of the ancient times and modern times; a man has to Go On A Quest [in the London Underground] after saving his true love; a boy goes wandering in Faerie to find a fallen star, to give to his beloved.

Neil Gaiman is using old legends in new ways, Tim Powers is making up new ones. Tim Powers is usually denser, having several important characters that interact in various ways, and usually have to join forces to defeat Evil. Neil Gaiman usually has one important character, although there will be other characters that are interesting, but never as fully fleshed out.