Food in AC biz is infinitely better than UA slop. Wines not so much. I find AC cabin staff much friendlier than UA's. Transiting YVR is horrible until you know your way round the airport. After that, It's not a problem. I would always book AC biz in preference to UA Polaris if the price was that much lower.
I don't find that's the case. AC food is maybe marginally better but not really. They don't offer hot pre-arrival meal on their westbound TATL. UA drinks are better in terms of quality and selection.
Food in YVR SS lounge is better than UA Polaris lounge. I find Polaris seat and amenities better than AC.
Transit in YVR is pretty straightforward, same as YYZ/YUL, going Transborder->Intl does not require any formalities or security. Intl->TB will go through security check and US pre-clearance.
But if price is cheaper and schedule is largely the same, then fly AC and save some money.