Going for the 22 KW unit may be the best choice depending on the $$$ differences over the 20 KW.
Based on the permits I process, 22 KW units are in the majority, followed by 20s, and 16s are few.
Common is a 200 ATS, 22 KW Gen, 100 gen, 200 utility, to the existing 200 amp panel (w/neutrals/grounds separated) and load shed for the HVACs, etc.