I'm still waiting to sample a 7-seat X. I found the 5-seat 2nd row bench to be quite comfortable, especially in the reclined position. The 6-seater looked perfect to me on paper, and it does feel more airy and spacious, but it's a little uncomfortable angling my legs towards the center aisle when in the 3rd row. For a dog though, I think that open aisle is great in the 6-seater. As I understand it, the 7-seater's bench reclines just as much as the 5-seater, and the 7-seater bench can slide fore and aft to reproduce the same amount of legroom as the 5-seater. I found the bench seat more comfortable than the 6-seater captain's chairs because the monopods barely have any tilt/recline.
Personally, I would go with the 7-seater. I noticed the 5-seater's bench allows you to fold just the middle section, so that sort of creates a more open space similar to the 6-seater; I assume the 7-seater bench has the same 40/20/40 split option but I've yet to test it. The real appeal is I simply find the greater recline of the bench seat much more comfortable than the 6-seater monopod seats, especially for extended trips.