ICE-21 vs ICE-20

The Cuisinart ICE-21 is a newer version, it has the similar features as the ICE-20.
However, there are some design changes to the appearance and the mixing paddle.
According to users (people who own ICE-20 and ICE-21) reviews, the key difference between Cuisinart ICE-20 and ICE-21 is the paddle design. The manufacturer claim that the new mixing paddle design on ICE-21 makes processing time 25% faster.
An user also points out that only the freezer bowl are interchangeable, other parts are not interchangeable.

For the design difference and recipes, you can download the manual to get details:
Official Cuisinart ICE-21 manual download link: http://www.cuisinart.com/share/pdf/manuals/ice-21.pdf
Cuisinart ICE-20 manual: https://www.cuisinart.com/share/pdf/manuals/ice-20.pdf

Some difference between Cuisinart ice-21 and ice-30BC:
The ice-30BC is larger, holds up to 2 quarts that is 0.5 quarts more than ice-21.
The ice-21 is sold for $30 cheaper than the ice-30BC on cuisinart.com
Design of the On/Off control is different.
Freezing Time on Cuisinart ICE-30BC is shorter.
The Mixing Arm is redesigned on ice-30BC, but it is small, only scrapes one side while instead of scraping both sides like the ICE-21, thus leaves more buildup on the sides.
The ice-21 produces less ice crystals.
ICE-30BC is noisier.

Compare Cuisinart ICE-30BC and Cuisinart ICE-21

	Cuisinart ICE-21	Cuisinart ICE-30BC
Price	$59.95 at cuisinart.com on 2014-07-16	$89.95 at cuisinart.com on 2014-07-16
Capacity	up to 1.5 quarts	up to 2 quarts
On/Off Control	Switch	Dial
Freezing Time	between 16 hours and 24 hours	between 6 hours and 22 hours
Weight	not specified	13.5 pounds
Dimensions	not specified	8.25" x 8.00" x 11.25"
Freezer Bowl	Double Insulated Freezer Bowl	Double Insulated Freezer Bowl
Feet	Nonslip Rubber Feet	Nonslip Rubber Feet
Cord Storage	Unused cord can be stored into the base.	Unused cord can be stored into the base.
Warranty	Limited 3-year warranty	Limited 3-year warranty
User Manual	User Manual	User Manual

Overall, according to users’ experience and reviews, the ICE-30BC is easier to use, but the ICE-21 makes better ice cream.