For me, it doesn't matter what country my cartoons come from. What I want is a good narrative with strong characters, just like any other narrative medium.
In terms of what I've seen, most western animation wants to be Looney Tunes. There's an assortment of "Whacky" characters, sometimes talking animals, sometimes not, who will constantly find themselves in physics-defying situations that should, by all rights, be terminal, and yet they always just get up and dust themselves off. There's no real plot beyond a static premise where any shake-ups are undone by the end of the episode/short, and the characters never, ever, ever, ever learn anything, grow as people or stop being idiots.
There are exceptions, and most of them are glorious (i.e. Beast Wars, ReBoot, Ben 10 (The continuity hiccups in the transition to Alien Force not withstanding), the Spectacular Spider-Man, the aforementioned Avatar: The Last Airbender, Batman: The Animated Series and it's successor, Batman Beyond...)
While Japan has it's share of Status-Quo dictated stupidity, as far as I can see it's outnumbered by series' where there's an overarching story, characters at least take a crack at developing and it eventually does end. This is even true of the stupidest anime I've ever seen, the Dragon Ball series (Note that I'm saying they're stupid, not bad. Although there could stand to be less talking and power level/dick-measuring nonsense and some more strong female characters. For God's sake, Naruto even had a stronger female cast before Kishimoto realized he can slap the brand on anything and it'll make craploads of cash and decided to write whatever the Hell he wanted.)
So I guess, as a whole, I prefer Japanese animation, but only because the Japanese seem to take animation more seriously as a medium, whereas here in the west it's seen as a babysitting tool.