If you are just digging 1 reasonably small hole & not needing to reposition the machine, both will dig based on the machine weight. The excavator is going to excel at digging & moving. But you are only digging 1 hole, so not really digging & moving simultaneously. A tired machine will literally drive circles around an excavator. The excavator is going to be a rough ride topping out at 2mph while the tractor is cruising at 15mph or so.
Presumably you wont be unloading the excavator right at the digging spot. So it will be a slow treck off the trailer to the dig spot(s). Then you sit In 1 spot digging a small hole. Possibly repeating that after moving a fair ways. The excavator will dig better, but I'm guessing you'll spend more time driving with either machine than digging. In which case the tired machine wins.
Not to mention the extra maintnance of another machine.