Tomo, the two stories are very different stories. Whisper is one of those "slice of life" stories, The Cat Returns is a pure fantacy, based on characters from Whisper of the Heart.
The Cat Returns was a Studio Ghibli movie, but Miyazaki let his newer crew work on it, with his supervision. So in that sense, it's not a 'Miyazaki' film.
It would be so hard to compare the two. Like asking which is better apple pie or chocolate cake.
Whisper of the heart is richer animation, a more emotional story. I think you might be able to easily empathize with the young girl, looking for teenage love and searching in herself for who she really is, and what she wishes to be. Actually, an old and simple plot, that always has so much emotional depth. You'll like it.
The Cat Returns is a simpler art style, but is supposed to be. it is a fantasy, with much more humor and adventure. Light hearted for the most part, but with some pretty good little morality lessons underneath. The girl learns to have confidence in herself, to accept herself as the good person she is. The plot is simpler than Whisper, but a lot of fun. I like them both very much, but for different reasons.
None of the studio Ghibli works are overly complex plots. They are generally made to be family movies, and suitable for most younger viewers. Even Princess Mononoke, Howel's Moving Castle, and a few others, aren't too deep a plots.
People involved in the industry take this subject very seriously. You should watch this. Besides, Micheal Colman played devils advocate, and gave Greg Ayres a tough time in this two hour discussion panel.
at least anime is doing better than american cartoons today. the only american cartoon thats really good is avatar but thats might as well be consider an anime too.