I would stay with Air Canada.

I used Aeroplan points 2 years ago to fly YOW to Kona business class and fly YOW-YYZ then YYZ-SFO and then onto a United flight to Kona. We found that Air Canada was way better. First of all, we had a two hour stop-over in San Fran which turned into 6 hours. We weren't initially allowed into United's Red Carpet Room (their version of the Maple Leaf Lounge) even with business class tickets. They eventually allowed us in only to find very little in terms of food and you have to pay for your beer. Funny thing was they had a sign in there bragging about being the best on-time major carrier yet most of their flights on the screen were delayed.

Also because they charge for every bag, most customers had huge carry-on bags so you might get stuck shoving your own carry-on under the seat and you'll lose some leg room. This is probably a moot point because AC is probably charging a fee for every bag to the US now too.

And the food on AC was far superior to the United food too. We had breakfast, lunch and dinner served both coming and going and each time the AC food was better.

The United plane was an ancient 757 with tube screens that come out of the ceiling. Most if not all of the AC planes have personal tv screens in the seat back. Even the cockpit looked old compared to the AC planes. Incandescent lights all over instead of LEDs and glass touchscreens on the AC aircraft.