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.