I confirmed my order in December for a 90D with 7 seats. Haven't had the opportunity to sit in an X yet. I have three kids -- two 12 year old girls who are 5'8" and 5'10 and a 15 year old who is 5'11". All have slender builds. Originally I was going to replace my 2011 Honda Odyssey with this car. The girls often sit three across in the Odyssey, even though the middle seat is quite small. That said, I rarely ride with the entire family these days, and often find myself in the car only with the twins. As it turns out, I'll probably keep the Odyssey as our "sports" car for going to travel softball tournaments and basketball games. It is helpful to have a "sports" car that can deal with all the equipment, dirt etc.
The theoretical rationale for the 7-seater is that it can accommodate more people in a pinch, and that one of the twins often likes to sit in the middle of the second row when it is just the three of us (despite the much less comfortable middle seat in the Odyssey's second row) just because that person can see better, which makes her feel less car sick and more able to socialize with the front row occupants. Also, with the big windshield of the X, the middle seat might be quite cool provided you aren't sandwiched between other people. In other words, I see positives when we have 3 occupants (one driver, one front passenger, one middle second row person), and marginally for 5 occupants (nobody is forced to sit in back row and we can use the expanded cargo space, although the girls may feel squeezed in the second row), or 7 occupants (only possible with the 7-seat configuration). It is pretty neutral for four occupants, and clearly negative for 6 occupants since at least one person will be forced to sit in the back and there will be less leg room for the third row in the 7-seat configuration. The frequency of having 6 occupants will be much lower than having 5 or fewer, and the utility of having 7 occupants probably outweighs the discomfort for 6 passengers. Logically, this supports the 7-seater.
It will be interesting to see how I feel when I actually get inside an X.