i'll bite.
Mule: Requires food, water, and has the potential to get scared in combat or make noise when it should be stealthy due to being surprised. Also surprisingly vulnerable to lead bullets.
Robot: requires maintenance, can resist bullets, requires recharging, and does not tire.
Lets be generous: Food, shelter, drugs, etc, to keep the mule healthy would be about equal to maintenance on the robot.
I'm being generous here, any sufficiently mass produced and sufficiently hardened military hardware requires surprisingly little maintenance(compared to some commercial counterparts)
Mule has a lower upfront cost, but lasts less time(old age, injuries, etc). However the robot, while being expensive, would drop in price as more are brought into service.
Likewise, robots cost little to store when not in use, and are quite compact. Mule's require a lot of work.
So yeah, local rented mules are great for our current situation, but in the long run (which is the military's main focus) and in other theaters of combat, the robot is a little more feasible.