I agree with Annie, it’s probably going to be the same airline no matter which you decide to give your money to.
I’ve been a “Super Elite” member of Air Canada’s Aeroplan for many years (down to just “Elite” this year, though). Almost all of my flights are Toronto/Toyko, with the odd Toronto/New York or Toronto/London trip thrown in. On occasion, I have flown United, including a a disastrous return from Tokyo a few years ago and quick trip to Chicago last week.
Generally, I find that Air Canada offers better service and more flexibility. United is decidedly average, although I’ve had enough experience with other airlines to know that I’d still put them ahead of a lot of their competition.
Just got home from Ireland night before last flying Air Canada. To be precise, due to the fun of connections I flew bmi from Dublin to Heathrow,and then Air Canada from London to Toronto, and then Toronto to Saskatchewan. I will preface this by saying that I have not been an Air Canada fan. In fact, flying domestically I have gone to fairly great lengths to fly WestJet rather than Air Canada. (I still prefer them due to the friendliness of service, and the fact that they do not oversell flights. On WestJet, if you have a ticket you have a seat on the plane.)_
The Air Canada portion of the flight over to Ireland and back was really quite good. On the flight back, I was on a 777 that was only six months old, and was spacious even in the economy class. All the seats had the little screen in the back of the seat in front, from which you have a menu of movies and tv programs that you can select. On the trip back, I would even go so far as to say that the food was quite good, and they kept coming around to see if passengers would like something else to drink. The flight attendants were friendly. I even lucked out and was the only person in my rpw, so I could flip up the arms on the seats and stretch out.
Believe me, I wouldn’t have been saying this all that long ago, but I can actually recommend them as a good choice.
Be warned, though: they do oversell. Don’t be late.
That is not correct at least for the Asian legs. United does not fly Vancouver to Asia. the Hagair’s hub is SF. Same for Northworst except all their Asian flights go through Tokyo. And you never have a case of buying a United ticket, showing up at the airport and being on a AC flight.
I’ll confirm my memory on Monday.
This was a few years ago, (back in the days when one could bring their own fluids on for in flight consumption) and it was some Yank airline with blue in the logo. Shit dude, cut me some slack! 
I will never fly United again after the last time I did – not that I was a loyal customer before – and the seats were impossibly close together. It was claustrophobic in my seat. Horrible. Worse was when we were waiting to board and they started trying to upsell us to slightly better seats on the exit rows (which I can’t take anyway because I couldn’t handle the doors) for an extra hundred bucks or so. I felt incredibly insulted, like they deliberately stick people in shitty seats (I didn’t realize how shitty at that moment, but still!) and then extort extra $$$ for what is supposed to be a slightly better one, and we’re supposed to think this is a good deal? How stupid do they think we are?
Remembering that still makes me mad, and it was a couple of years ago. Jerks.
I rarely fly, but Air Canada seems to have improved in recent years (as K364 says, probably due to competition). IMHO United Airlines could be a lot better, as I recently described in my pit thread, “United Airlines could be a lot better.”
Not mentioned yet: Air Canada has free cocktails on overseas flights.
Then maybe he’ll get lucky and buy United, then take Cathay Pacific to Asia. They rock!
Last time I flew overseas/international with United they did also.
AFAIK, Cathay and United don’t have a code sharing agreement either. Eg, you won’t buy a United ticket, get on the flight, and then be pleasantly surprised that you are now on a Cathay flight.
Also, given that the OP wants to go to China, this won’t happen. Cathay has virtually no flights to China. IIRC, the few they do are all from HK.
no code share on shanghai to hongcouver. hongcouver to seattle is a code share with united.
service is better than united. my meal sucked. bastards got rid of the biz class desert cheese plate and apertifs.