I would go with AC as much as possible.

Assuming you are traveling on a paid fare you should have access to the AC Signature suite in YYZ which is much better than UA Polaris lounges.

Two more points:

Polaris Cabins tend to be bigger than AC Singapore cabins. I just flew AC last week across the ocean and the cabin was only 25-30 seats and felt quite private with amazing service.

In case of IROPS AC is going to do a lot more for you. Delays happen on every airline but there are laws that require AC to do a lot more for you. My last two delays involve AC and UA.; I had a UA and an AC flight cancelled this year: AC airport agents put me on a BA flight no questions asked but when my UA flight was cancelled the UA phone agents would not even consider moving me to another airline.

Safe travels!