FWIW:
I have a lot of time on both 26" wheels (on my mtb) and 700c wheels (on the road bike)
the road bike is great for sustained high speeds.
the mtb accelerates faster even with slightly heavier wheels/tires. Less rotational inertial, i assume. and it's not just a feeling: i've coasted down hills next to roadie friends, and i pick up speed faster. the smaller wheels just get moving faster.
since my commute has a LOT of stops/starts, my daily ride is the mtb.
If i had a lot of straights, I'd be on the road bike, as it's easy to cruise 3-4mph faster than the mtb. but the longest straight i have is less than 1/2 mile.