Unless there is an equipment swap, you will be on a two class plane in F. There are a few domestic three class birds (in addition to the JFK market), but those are getting rarer.
The seat on UA three class C is always better than two class F. The service is reported to be better in two class F than three class C. But I've never really though of it all that different. I would therefore go for seat over service.
That said, there are other options (if you are willing to change some flights)
1) Reroute yourself to LAX-JFK/LGA-YYZ. Mind you, you will have to change airports in NY, but you will get transcon PS F.
2) Reroute LAX-IAD-YYZ. If you don't mind overnighting in LAX, there is a morning LAX-IAD on a three class 777.
3) Reroute LAX-YVR-YYZ on AC. I think there are still some international C products on the YVR-YYZ sector.
4) Catch a nonstop on AC LAX-YYZ. It would save a lot of hassles, and the service will be just as good, if not better, than the UA F (two cabin) flights.