Air Canada International Economy vs United Airlines International Economy
 

If there is a strong desire to fly only Star Alliance airlines, and on that international route there are only two choices, Air Canada or United Airlines, which will be "better" in coach?

Depends on the aircraft, as they have different seating configurations. In general, AC has high-densified most of its long-haul fleet, whereas UA has only partially done that.

Depending on where you're starting/ending, there's also the issue of transiting another country. It's an added hassle, especially if you're Canada based, because the US transfer is a pita (Canada no picnic either).

777-200LR on Air Canada

787-9 on United Airlines

At least it's not either carrier's 77W (777-300ER). Those are to be avoided at all costs in economy.

Both of these cabins have been densified, though. The United aircraft will perhaps be slightly less bad, with a slightly wider seat and slightly more average pitch. The 789 has the higher cabin pressure (more oxygen, more humidity), which may make it a little more comfortable for a long flight.

Neither carrier is known for warm friendly service in economy. You're probably less likely to get an outright rude crew on AC.

To me avoiding an extra transit in either the US or Canada would make up for any minor difference in aircraft comfort.

How about in terms of catering?

It's economy on a North American carrier. It's going to be food most of us wouldn't touch on the ground. Eat before you board and bring snacks.

Both are horrible! No way around it.
United catering has been historically horrendous, but its improved a lot over the past few years.
AC catering is mediocre.
I'd say they are equal.

Overall though, those are 2 crappy options. Choose whichever gets you there quicker or costs less.

Red, what is the routing you're looking at? Do you have status on any *A carrier and if so, which one?

Planning to head to Vancouver and Seattle from Sydney. ex-SYD to either will be fine as I will probably either drive or fly domestic between the two. Unlikely to buy open jaw as I don't think is worth it. Would preferably like to accumulate KF miles. An alternative will be to fly Air New Zealand via Auckland. Do not have any status.

United.

Red, you can take Amtrak (train) between Seattle and Vancouver. Crossing the Canadian border to the US in a rental car can be a real pain in the butt, and while it is a short flight, the train ride is about 4 hours, and that's from downtown Vancouver to downtown Seattle. Getting through US preclearance in Vancouver airport adds an hour or so to your day, so it's probably a wash between flying and train.

Fro the flight, Air Canada does have a non-stop to Vancouver. No one has a non-stop to Seattle from Sydney, so you would need to connect in San Francisco or Los Angeles. Not a huge deal, but price each option out. Do not connect in the US to fly to Vancouver, since then you have to go through US Immigration and Customs, then do it again in Canada.

I have driven between US and Canada at Niagara Falls before and it was ok, though it might be different at this location. However, I have not firmed up how I intend to get between these 2 places yet. I plan to head to Boeing Everett Factory, so I can head there on the way to Vancouver, but will take what you said into consideration.

Amtrak has a couple of trains between Vancouver and Seattle that do stop in Everett. A car is probably easier in that case however.

The Amtrak train schedule between Vancouver and Seattle is terrible. The buses (Amtrak, Greyhound, BoltBus, and more) are generally quicker, cheaper, and have far better schedules.

I've never had any issue in either direction, though it may be different for an Aussie citizen.