I just faced this dilemma for an upcoming paid work trip. The on direct UA flight has terrible timing...gets in on the late side and leaves painfully early given that one has to do customs pre-clearance before the flight as well.

The problem with the AC flights is that it's really tricky to figure out how the AC fare codes map to UA fare codes as the Tango fares don't earn UA miles. After 3 calls to CS, I wasn't convinced they knew how these would map. AC doesn't give any seating preference to *G (not that UA does either), so that was also not so appealing.

I ended up doing UA through Chicago on the way there. As a 1P, I'll put my upgrade clearance probability at about 2%. Coming back through I'm going through CLE...I'm hoping I get a bit luckier with the upgrade on CLE-SFO.