As sillvgl mentioned, with only 1 kid, there's pretty much no bad option. The question you have to consider is how many kids you will have (during your ownership of the X). I have 3 - 2 forward facing and 1 rear facing in a 6 seat X. My thoughts:
1. Unless you will have 5 kids constantly, I don't like the 7 seater. It's pretty cramped back there, and not easy to get to.
2. With 2 kids, both the 5 or 6 seater work well. When I had just two, they'd be in the 2nd row and the third row would be folded down. During stops, they LOVED climbing all over the insides. The folded 3rd row/trunk would be their play/picnic area. At supercharger stops, we would turn on child door locks, unbuckle them, and let them free in back. You can do this with a 5 seater, but they wouldn't have easy access to the back.
3. With 3 kids, I would say the 6 seater is best option. You're limited in the selection of car seats that can go 3 in a row to begin with. Then you have to deal with three kids in close proximity hitting each other. I value my sanity and space them out a bit. You could do this in the 7 seater with the third kid in back, but as I mentioned earlier, getting in there is difficult.
4. I use combinations of Britax Advocate (rear facing) and Britax Pinnacle (forward facing). They are big, but still fit. Rear facing can only fit in the 2nd row, and they limit the leg room of the 1st and 3rd rows.
5. You can get two forward facing in the third row. I did this for a while. The advantage is that this opens up 2nd row seats for adults. The disadvantage is the kids are in close proximity to each other, they're pretty far from the front so you can't assist them, and it limits their play area.
6. My current layout is rear facing in driver side 2nd row, and 2 forward facing in passenger side 2nd and 3rd rows with the eldest in back. The driver side third row is folded down. I can reach the two smaller ones from the front and the eldest enjoys her "table" next to her seat. At stops, they can climb around in the 3rd row/trunk. I bring compact lawn chairs for the adults, so we can even open the trunk and watch them if the weather is nice.
I can't stress enough the value of letting the kids roam free during stops. It's not realistic for them to be strapped down for the entire supercharger stop, and there are times (basically most of the time!) that I don't want to have to follow them around outside the car. Inside, they can play board games while I rest in an air conditioned environment. Yea, that's worth $6k to me.