Short answer: it depends.

Based on the SFO fleets you listed, 737 has the most diverse flying and is a growing fleet. But you can expect to fly 2-3 legs per day on average, and quite a few red eyes.

A320 is basically RJ flying; lots of short legs (I believe 3-4 legs per day, someone correct me if I’m wrong) and not too many diverse locations. Hope you like Des Moines!

756 SFO is a dying fleet, but United keeps filling lots of NHs into 756 SFO. Your relative seniority in base will be faster (for the time being) in the 756 compared to the guppy or bus, affording the possibility to hold commutable lines quicker. Hopefully you like flying to Boston or Chicago all month, because 756 SFO is almost entirely transcon, with a handful of Hawaii trips.