I don't really like to say which is better, because they're so different. They have completely different styles and have different strengths.

Like, Disney's great for a good old feel-good cheese-fest! They have better songs, and they're usually not too heavy. Even in the Lion King, which is generally considered to have Disney's saddest moment, the sad part is swiftly followed by chirpy songs and funny jokes.
But a Disney has never made me cry, where a Bluth can.

Don Bluth films are just so beautiful. The art is usually amazing. They tend to be led by the emotions of the characters, while a Disney is usually more powered by the narrative.

In a Disney someone is always either talking or singing, but Bluth films have these quiet, enchanting moments that Disney just can't pull off.
Like near the end of American Tale, where these brightly coloured little birds play in a bit of water. It has nothing to do with the story, it's just a lovely little piece of film.
There's a similar scene in Land Before Time, when some little "flappers" fight over a berry, then at the end one of them offers his berry to LittleFoot. It's actually quite moving, but not too over the top.

I think that's the main difference; Disney films are more fun, but Bluth films have more charm.