Depends on what type of rental your referring to. Some places rent out those small stumpgrinders that you push around like an oversized walk behind mower, and some rent out legit commercial machines.
If your talking about the larger commercial machines then they would make quick work out of those stumps. If your referring to the smaller homeowner ones I wouldn't even waste your time.
The larger (worthwhile) machine might cost about the same as hiring it out, so you may not save much money, but at least you'd be doing the work yourself like you want.
I grind stumps as part of my business, and it's typically less money for me to do the work for the customer vs them rent and do it themselves.
I just did 2 jobs, one grinding and one chipping, and it turned out to cost the homeowners the same amount of money if they had rented the equipment and done it themselves.
(And that's using average wages, not working for pennies)
If you can find a good rental, go for it! It's very satisfying to demolish a stump into nothingness
