Originally Posted by
flyquiet
Not sure how lounges factor in. If you are E50, can you not use the MLL in YYZ even when flying UA?
Yes, absolutely, but the OP would have to set foot in YYZ to use the MLL there. OP wouldn't transit YYZ if flying UA. I believe the OP's route would be YXU-ORD-LAS.
So the advantage to flying AC would be the better lounge in YYZ. The advantage to flying UA would be pretty much everything else.