Having flown both AC and UA recently in Y on this route, I would definitely go for UA if the price were the same.
UA Pros:
- Friendlier and better service
- Free advanced seat selection (for my itinerary)
- Way better on-time performance for this route
- Stroopwafels?
- Reasonable upgrade pricing to J (800 CAD vs 250 USD IIRC)
Seats felt about the same. The only thing better on AC was the paid bistro service during my flight for hot meals.