I've been around both planes since that were introduced.

They fly enough alike to be considered in the same category but the UCD is much smoother because of the longer tail moment.

The Twist is more durable

The Twist is cheaper

The Twist has an upright open engine which is more convenient and trouble free.

Use large engines in both.

The UCD 40 likes a Saito 100. Fits like they were made for each other
The UCD 60 likes the Saito 125 or OS120AX

The Twist planes like same engines