I'd go back to trying out with seats the models your interested in, of course letting the primary driver decide. when it comes down to it, how long do you keep vehicles? If not more than 5 yrs, then choose whichever you like best for the seat set up you like best. If you keep em 5+ years, then your decision may not depend so much on kid size short of them today physically fitting as over the next 3-4 yrs, the twins will grow past the simple seats.
The note on service, well, I agree ... if you choose a Mazda 9, then you may not need to worry much on a local dealer as it's unlikely you will need one. However as noted too, minivans really (to me) are a nice option as one needn't climb up into them for seating.
Overall, my opinion is what are your primary needs for the 95% of the time? If you only need the seating for 2-3 times a yr, or 1 to 2 times a month during ski season ... why worry? We haul our kids two to 3 times a year on a 13hr drive to Co ... in a crew cab pick up .. 3 kids wide in the second seat. We've survived, so overall, is it a significant issue or simply a wish?
On mileage, minivans get greater mileage in general. Like Dad, we bought a 2000 GM but we did/do get 25mpg which is much better than the 20mpg the crew cab gets. However the Crew has 4wd .... but as a friend noted, we only need 4wd 2-3 times a year and in most cases, one day makes the only big deference.
So, I'll go back to choosing what you want over what you think you may need, it really may not matter too greatly over a second choice.