I've played L'isle Joyeuse in the past and I'm currently working on Polonaise-Fantasie. To me, it seems much of the Chopin fits under the hands quite well, and quite a lot of the piece is not particularly difficult to get going. That said, there are tricky moments, and I've a lot of work to do yet before I have it truly under my belt. The Debussy by comparison, seems a bit more relentless in the way technical difficulties are posed. In terms of musicality, I suppose the correct answer would be that they both require a lot of preparation, and I guess this is an even more subjective area, but again I would favour the Debussy as being more of a challenge.
But, bear in mind that the Chopin is about twice as long as the Debussy, so there could well be more work in the Polonaise.
By the way, I'm Richard - I've been loitering in the background for a while, absorbing some of the fascinating discussions past and present. What an excellent discussion forum - it really does seem to be everything I could ever want in a forum on this subject. I thought it was about time I made a contribution.