If you use UA you can go with a 777 from LAX... and Denver.
United J class has better seats but food service is not as good as AC.
If you can do a star award, go with the UA 777 one way and come home with AC on the other.
Make sure you get AC international J class or you will be... unhappy
Look to discussion on FT at the moment on the domestic YYZ-YVR flights.

It will be fun to use both airlines but I think Star awards have more restrictions.