I'd definitely take the 22 AA. Thats a huge improvement over the 20 AA.
Example of a 20:
Bio - 18
Chem - 19
Orgo - 19
RC - 20
QR - 22
TS = 19
Example of a 22:
Bio - 20
Chem - 21
Orgo - 21
RC - 22
QR -24
TS = 21
A 2 point increase in AA means, on average, that each component was raised by 2 points. Big difference.
Plus, the PAT is important, but not THAT important. For a school like UOP it is important, but for most other schools, anything above a certain threshold is acceptable. And 18 is definitely acceptable.