I think that you will find the primary difference between the Duo and K-21 is energy management. Teaching students to manage energy and land is one of the keys to getting them solo in a reasonable amount of time. My club purchased a Duo XL last year and while we have been training in K-21's for 10 years, the transition to the Duo was not easy for some licensed pilots.

It is much quieter than the K-21 so the noise feedback for speed control is gone.

It is much slipperier than the K-21 so subtle changes in attitude result in much larger changes in speed. Couple this with the lack of noise and pilots were finding themselves over 70 kts on final.

It carries more energy into the landing than does a K-21, although the addition of landing flaps to the XL version resolves much of this.

Besides its handling and performance, another nice advantage is the sideways opening canopy does not result in burn marks on the instrument panel in the front and headrest in the back. We've sent an S3 vario back 2 times for screen replacement because of sun burns that happen almost instantly when the sun is in the worst possible spot for a K-21.