Will all of you always be traveling around together or would it help to have multiple vehicles so you can split up into two groups?
If you always want to stay together no matter what than get the mini-van. If you suspect that there may be times that you want to do separate things than get the two cars.
Depends on the 4 adults. If it is grandparents, I would go for the mini-van. It is so easy to just hop in one car to get any where.
I would put the kids in the third row of the mini-van and 2 adults in the middle row.
We had 7 of us in a mini van over Christmas- I thought about getting 2 vehicles but did not want to pay for parking for 2 vehicles each time- we fit okay- even had 8 suitcases on the way home- everyone had to have their carry on bags on their laps/at their feet but since that was only a few trips to the airport and back it was not too bad. 5 of us were "adult" sized- being 3 teens and 2 parents...then 2 seven year olds. 2 vehicles would be really nice for more space and if you were going to need to go places separately.