This is really an unanswerable question. Therefore I wont fall into the trap of saying one is "better" than the other but instead will state which I prefer. I have played a lot of both composers' music, french "impressionist" repetoire is among my absolute favourites, I have to say that it is a hard decision but I would probably go for Debussy. Having read a fair bit about his life I do find him a fascinating person. For example the story about when he first entered the conservetoire in Paris and amazed everyone by being able to create the sounds of city life and traffic etc. at the piano. He was also quite rebellious at times and I get the impression that he lived in his own little wrold a lot of the time.
Debussys' music was revolutionary and was maybe the first real step away from romanticism. It is interesting to note his taste in music as well, he liked Wagner but didn't want to compose like him but at the same time he admired much earlier composers such as Palestrina. Although not always obvious the influence of such music can be heard at times in his music. Ravel was very much influenced by Debussy, by the time Ravel became an established young composer Debussy had ben around for some time. The fact that two great French composers were composing at the same time caused the press to create the idea of a rivalry between the two.
Debussy, in his earlier years as a composer, had to cope with a public that maybe were reluctant to embrace his new approach to such musical elements as harmony and form. It took people a time to understand his music. This is one reason why I think that maybe Debussy is unfairly dismissed, because people don't know how to listen to his music and thus don't understand it. I had a similar difficulty when I first got interested in him, however once you get used to his style it opens up a whole new world of amazing music. Most pianists will know pieces such as clair de lune and Reflets dans l'eau, but how many other pieces of Debussy have you actually heard? What about the images, what about the preludes, what about the orchestral pieces such as prelude a l'apres midi d'un faune, the images, and the incredible la mer? All of these pieces show an amazing imagination and a breathtaking subtlety and attention to detail that not many other composers come close to.
I get the impression sometimes that a lot of people base there perception of Debussy on the experince of one piece that they heard or had to play that they didn't really understand and therefore this has put them off him for good. Yes I admit that he takes some getting used to, there aren't really any tunes as such (not like in say alborado del grazioso) and this takes some getting used to. However it is not tunes that he is wanting to write, his use of harmony is so beautiful and subtle. For example in the second book of images for piano he displays his full range of colours and imagination, from the subtle suggestion of moonlight and sheer beauty of et la lune descend sur le temple qui fut (one of the most magical and hauting pieces ever written for piano, as well as a piece that has the most beautiful use of harmony) to the playful and almost jazz harmony of poisson d'or.
Of course Ravel was a genius as well and wrote many beautiful pieces such as the miroirs etc. however I think I prefer Debussy and I think he is often overlooked.
(sorry I took so long to say that!)
Topic: Ravel or Debussy?