Agree 4t's are great, reason for my user name! The CX9 replaced our family's last remaining non 4t vehicle. We have five of various ages.
I don't know what it is about 6's but I've never had any luck with MPG driving them. IMO may as well have a v8 if you're going that route. Modern v8's are nothing like the old ones. From friends I've spoken with, their v8's net about the same MPG as my 6's. I've driven a flat6 an inline6 a v6.. all disappointing mileage for what they were.
Look at the new Atlas from VW.. Big 3row and what did they decide to offer it with.. a 4t. And I'll bet you it will haul plenty well enough with it. A lot of people gripe about Mazda's sticking solely with fours, but I'm all about that.