Some comparisons if you were to commute tomorrow -
SLC to SFO - 9 flights (7 mainline). Of those flights, only four have UAL home team advantage (including both RJ flights). About two hours gate to gate. OAK and SJC are viable backups when WX goes down with reasonable public transit options for getting to SFO. Need to cover SJC trips too. Lots of UAL commuters.
SLC to LAX - 11 flights (9 mainline). Only two UAL flights, and they are both on the RJs. Trip time also about two hours gate to gate. SNA/BUR/ONT are not great commute backups because of the need to Uber and traffic (and you might have to cover SNA or ONT trips). Likely to be a fair number of other offline commuters from every airline to compete with.
SLC to LAS - 10 flights, all mainline. Trip time about 90 minutes. No other nearby airports to commute in to, but no other airports to cover either. Fewer commuters, probably mostly WN and DAL ones.

If waiting for DEN, you should seriously consider LAS. The trip pool and schedules are not great but your seniority will be at least a match with SFO.