#20 is one of Meg's "older" recipes. While it lists as a sealant, #20 does have some slight cleaning ability. Almost like a real light AIO. #21 is a pure polymer sealant with no cleaners. I have used both am happy with both equally. As a sealant, #21 is pretty awesome though, so I might favor that on a finished ,prepped surface.
#21 has stronger cleaners than NXT. Go right to the source for more information:
"Meguiar's #21 is a synthetic sealant in that it is totally man-made. Moreover, Meguiar's #21 takes a no nonsense approach. It is purple in color and smells very similar to #20. It does not have a fruity or otherwise catchy smell. Instead, it has a mild chemical smell not unlike #20. As a result, you can tell right off that this product was meant for the professional! It is a thicker "wet" liquid. When I say, "wet" I am referring to its spread ability and its ability to stay wet when used with a rotary buffer. Hence, the reason it being geared towards the professional detailer. It also has mild specialized cleaners; slightly more than NXT Tech Wax but less than #20 Polymer Sealant. However with that said, if there are cleaners, then they are extremely mild; please see text below."
I found 21 to be a little bit darker....other than that, they are both easy to use (applied thinly) and look great. Auto Detailing solutions has both in smaller sizes if I'm not mistaken.