I am flying Premium Economy PHX/ATH.
Air Canada rt is $400 less than AA rt.
Assuming one airline is better than the other, which airline would you select?
Also, does having Global Entry expedited my transfer in YUL going to ATH?
Thank you!

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.

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.

I did USA-YUL-International a few years ago and while I don't remember if there was a passport check, it wasn't more than just a cursory glance and direct to the international gates. Certainly not the full immigration experience. And YUL does have Global Entry for the return when you pre-clear into he US.

AC's on time performance has been poor post-covid. I'm not sure if it's improved in the last year. I would probably fly AC given the savings and equality of PE offerings across airlines, although I would do three things to ensure my trip has some resiliency against misconnects: not schedule tight connections (allocate at least 2-2.5h), buy with a credit card with good travel insurance, ensure these are daily flights.

If the alliance doesn't matter to you, then I would recommend saving the money.

What's the full routing? A large portion of your trip is going to be in economy PHX-YUL (and maybe onwards if not going YUL-ATH).

Echoing the full routing question, with an add-on. Are you already familiar with the connecting airport(s)? To me that is worth something.

Is there an intermediate stop either direction you would like to make?

But regardless of the routing the same is true for AA, so this doesn't really move the needle at all IMO.

On AA it would presumably be PHX-ORD/JFK/PHL/CLT-ATH or even PHX-LHR-ATH. The shortest AA connecting flight option (ORD) would only be about an hour shorter than going through YUL.

Not something I would spend $400 more for, but to each their own.

Yes and no - I'd say it's better to have the segment in Y first then cross the Atlantic in PE vs the other way around.

Wouldn't the TATL segment be the one in PE, regardless of whether the carrier is AA or AC?

By the other way around I mean do PHX-LHR in PE then LHR-ATH in Y

Oh, yes. And Y would be especially bad on BA short-haul aircraft, with 28-inch seat pitch!

Thanks for the heads up on Global Entry!

My experiences on AC, and I have many, mostly pre-pandemic, have been pretty awful. I would put AC operationally well below the worst of the US airlines (currently IMHO JetBlue). Flights are chronically late, staff is often clueless when they are, and when something goes wrong they are terrible at providing help (think lines of people in a terminal served by a single agent and no ability to rebook by phone). I'd take AA in a second, though as you can see there's no case I'd take AC unless I had no other option. Some of this may be informed by their terrible service in BOS; in other cities where they have more of a presence it might be better. And take it with a bit of a grain of salt because it's been a few years since I flew them regularly...