They both look exactly the same in function, its just the 21 is a bit smaller and has more plastic. Check this guy's video on Youtube and it breaks down the ICE-21. http://youtu.be/jPPdSCHBTR4 And he did the same for the ICE-30BC: http://youtu.be/Q40vHKdJmpY
Never used the -21, but the -30 is very easy to use. Freeze the bucket, put it in the machine, add the mixer, turn it on and add the liquid. Done. Clean up is easy too. Just remember if you are adding alcohol add that at the end or the mix wont freeze right (yes even vanilla extract will cause the mix not to freeze right).