Originally Posted by
bscooter26
Global Entry is not accepted when entering Canada. However, YUL does have a US preclearance site, so you would go through Canada customs and immigration then US customs and immigration on your return from ATH in YUL, rather than the latter when you land in PHX.
Connecting from International>USA at YUL, there is a direct path to connections and the US Pre-Clearance site, so you do not have to clear Canada immigration/customs. I'm 99% sure the same is true on the outbound USA>International, direct path to connections/international so no need to clear CBSA.
For a $400 savings I would go with AC, all else being equal, and assuming OP you don't have or are chasing AA status.