Tough question. Both extremely high quality, beautifully drawn Japanese anime films from Studio Ghibli. Whilst Spirited Away did get the oscar nomination and is at #46 in imdb.com's all time best movies list, I think I prefer Princess Mononoke which comes in at #118 on imdb.coms top list. Maybe I need to watch Spirited Away again, but I did get big lost off at certain points, like when the sorceress' twin sister turned up to complicate matters.
I guess its a personal choice which you enjoy most! My only advice is for people to hunt these films down and watch them. They make Disney look like a bunch of amateurs. :)