I own both.

Two different flying airplanes. They both do many of the 3D tricks well, however there are some differences.

I like the Flip better.

The UCD is more stable, flys similar to a pattern plane.

Meet me at the field some day and I can tell you the difference between the two, not sure I could accurately do it in writing. That, and you can't see my hands move when describing flying them.

Hardcore 3D: Flip

Learning or want to learn 3D: UCD