Welcome to FT.

First, I've never flown AC. However, I know the business class seats are similar. So, here's my opinion

* YYZ-YVR is on the same aircraft as YVR-SYD. So you have international J seats all the way. On DL, you'll have to slum it on the domestic F seats between DTW and LAX.
* You have to deal with US customs, immigration, and TSA on DL.

==> Stick with your nation's with your home carrier (and CATSA.)