HA! I wouldn't be packing stuff for anyone over 12 unless they had a disability, were pregnant, etc. If you can present someone with a perfectly reasonable alternative and they don't want to participate, they should get all the opportunity possible to "enjoy" the results of their philosophy :)
My wife is an experienced hiker and interested in saving weight in things like cooking gear or a good tent, but she doesn't think much of tarps and clings to an ancient frame pack, no matter how much I have offered ultralight replacements. One thing about that old frame pack-- it is so old that it is a very Spartan design and the hip belt is just a web strap-- good for stabilization rather than weight transfer. To my complete shame, she wears jeans too :)
I guess "hike your own hike" holds at home as well as onthe trail :)
To answer your question, the smallest pack that will haul the weight and volume you need. The rest is what you want for bells and whistles at what weight penalty. I use a GoLite Speed with lots of pockets and padding, but a Gust model would haul the same amount at 12oz less. I recommend a GoLite Trek which will haul good volume-- lots of room for bulkier clothing, food and/or a bear can, but will still limit you to 30 pounds total.
On the marriage counseling side, try getting her to try *one* trip ultralight-- throw out the extras, Spartan clothing list etc. After going up a trail with a 20 pound load, I was sold!