It's looking like I'll be traveling quite a bit (around 2x/month) for work from NYC to YYZ for at least the next year. I'm also unlikely to have work travel within the US, FWIW. So I'm looking to decide between WS and AC. I've primarily been on WS so far this year, with one AC trip, and both seem to have their pluses and minuses. However, I'm going to try to request that I always be on the same airline, so I'd like to work out which one that should be.

I expect to be ex-LGA most of the time due to availability. For a 90-minute flight, I don't care too much about the minutiae of in-flight experiences, so I'm looking for more qualitative reasons to pick one or the other. I'm leaning towards AC due to superior interlining, rebooking, and mile transfer/earning opportunities on *A, plus it's nice to have lounge access on the YYZ transborder end (there's MLL, and I have access to Priority Pass via one of my credit cards as well). That said I've been perfectly happy with my WS experiences thus far (onboard experience is pleasant, and I've yet to experience a delay due to a WS-caused problem), and AFAIK my company has a better deal with WS than AC so it might be easier to request.

Any thoughts?

Not sure if you will have enough flights in the next month to qualify, but WestJet is currently offering a fast track to Gold for flights between YYZ/YOW/YUL and LGA/BOS

https://www.westjet.com/en-ca/reward.../fast-track-20

Before you commit, wherein New York are you and where in Toronto are you heading to?
There are 3 airports in New York and 2 in Toronto.

WS operates a very efficient turnaround out of the Delta terminal. Is there a PP lounge in that terminal? If so, I am not aware?
Over in the Central Terminal, you have a choice of three lounges (assuming you have the AMEX Platinum card?)
- Air Canada;
- United Club Lounge;
- AMEX Centurion Lounge (my PF!).
Recently, I took the MTA to Central, spent two hours in the AMEX lounge, and then took the purple shuttle over to the Delta Terminal. Very do-able.
Another advantage of AC option? Due to LGA const'n, they will change your ticket for free to EWR.

Unfortunately I won't hit 10 by the end of June, but thanks anyway!

In NYC I would prefer JFK (I have almost-direct subway access), but LGA has the best availability (I prefer pre-7AM flights so that I can get into the office by 9) so that's where I expect to be. On the Toronto end that seems to always mean YYZ.

The PP lounge at LGA is the MLL if I'm not mistaken, both in the central terminal. I have a complementary PP membership through my CSR, no Amex. Unfortunately the central terminal lounges are pre-security so I'm not sure how useful given that they usually open around the time I'm boarding as I usually take early flights out of LGA. That said, my WS experience at LGA has been very pleasant and I much prefer the Delta terminal experience there in general as well.

Given it some more thought and I've decided that AC has better benefits for my travel pattern (I should break 25 AQSes by the end of the year, 35 not out of reach) and the schedule flexibility helps.

One final thing - maybe a dumb question, I'm new to navigating frequent flyer programs - given that I'm US-based, is there any advantage to booking via United and accumulating points there instead of AC? I have no status with United so doesn't seem like a big deal to accumulate entirely on AC.

Go for United. AC is ditching Aeroplan and starting its own program in a few years. Aeroplan runs like you owe them something - you have to accumulate miles every year or you lose them all - not so easy if you don't travel in Canada. Aeroplan has very high fees and taxes and their cancellation policies are very complex (cancellation within 22 days of the flight costs big time). I hate the way Aeroplan is run. AC started AP and spun it off but it has been run to the detriment of the public and it looks like AC found a way to avoid the negative impact by getting out.

Who is buying the tickets?

If Air Canada, will you be able to purchase the more expensive Flex class, or like many business travellers will you be forced to purchase cheaper Tango class tickets?

If Tango, I would avoid Air Canada and Air Canada's program. It will be very difficult to earn any status or perks.

As others have said, I would seriously look at United. They have a very good program and it's fairly easy to earn status.

(At least until United adds "Basic Economy" (BE) that route and if you're expected to book BE like some business travellers are being made to do.)

...and on a tangentially related note, do you have your immigration / visa situation squared away? While the Canada Border Services station at YYZ tends to be more lenient to business travellers than ports of entry, eventually this pattern will cause increased interest.

Also, if you don't have it already, I would also strongly encourage you to get Nexus (which gets you Global Entry) which will make crossing easier and will also get you expedited security in Canada and TSA Pre✓ in the USA.