99.9% of the time the 15 year will pay off earlier even if you make the same payments as "extra" on a 30 year because the 15 year rate is significant enough difference typically to let a 30 year be compareable. Now when you get to much smaller mortgages...say 75k and under the payoff time difference becomes almost unnoticeable(assuming low rate environment).
WIth that being said the huge benefit of a 30 year mortgage is flexibility. You can paydown extra if you want, you can keep the high cash flow if you want and you will qualify for new loans easier if you get 30 year mortgages because your DTI will be lower.
The interesting idea would be to see what kind of principal paydown it would take to accelerate a 30 year amo into a 15 year time period so you'd receive benefits of low DTI while still paying it off in same amount of time(obviously only beneficial for a small group of people).
This all being said I keep rentals at 30 years and personal at 15...want to pay off my personal residence asap so if the unthinkable happens..major injury, death to wife or myself etc we'll be able to live in comfort on 1 income and keeping extras for kids futures.