Well coming from someone with a family of six which includes 4 children ranging from age 2 to 11, I can tell you that the Sierra Crew Cab was the best purchase for our family. We looked at a lot of 3 row SUV's, but we realized that the lack of cargo space did not fit our needs. We specifically ordered a truck with the front bench for the 6 seater capacity and the rear as more than enough room for a bulky car seat and 2 booster seat bases. Oh and best of all, the bed is perfect for all our luggage, bikes, coolers, golf clubs, basically anything we'd want to bring a trip. The wife also feels and probably is much safer driving it, especially in the winter.
As far as your question goes, if you have the funds to pay for the AT4 and that's what you really want then go for it, but in my opinion the "regular" Sierra is probably all you'll ever need. I mean red tow hooks, a 2'' lift and more aggressive tires aren't worth it for me. I already have most of the Off-Road stuff from the X31 package and you can just buy off-road tires down the line once your wear down the decent all-terrains it came with.