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.

What is your full route including connection(s) on AA?

The PE hard products are so similar you wouldn't really notice the difference, so the bigger question is whether the price difference and extra hassle of going through a second country's customs and immigration is worth it to you. $400 is not an insignificant amount of money to most people.