it targets a save which most monsters are worse at than fortitude
They attack different stats. Your assumption about reflex versus fortitude won't survive contact with the enemy. If you're battling a nest of basilisks, your party members aren't going to accept "but most monsters have a better fort save!" as an excuse when they are all turned to stone. You asked when one is better, and the answer is when their reflex is weaker than their fortitude.
also steals a movement action
Shove steals a movement action too, since the enemy has to walk back to you. It doesn't make them off-guard, but it does potentially get them out of position. If you have Reactive Strike and a Reach weapon, a shove can be devastating, particularly a 10ft shove where it requires a stride rather than a step to undo.
Also, since you upgrade success to critical success, that means you can push further. Positioning matters a lot in pf. You could fuck with the enemy's attempts to set up a flank or otherwise do something powerful that requires good positioning. Trip makes the enemy vulnerable, but Shove can be a defensive tool.