M20 is more an cleaner wax/AIO as opposed to #21 which is a LSP. here's the order of least aggressive to most aggressive in the megs cleaner wax's...
Meguiar's A12 Cleaner/Wax (Consumer Line - Maroon bottle)
M20 Polymer Sealant
ColorX
M06 Cleaner/Wax (ColorX, M06, M66 are all very close, would put M06 and ColorX on the same par, M06 can be applied by rotary, that is the major difference)
M66 Quick Detailer
D151 PRC