I like the durability of a heavier pack, the way it carries, how stable it is, etc.
However having said all that, there is something to be said for keeping pack weight down. It may keep your arches from collapsing and keep you from having knee problems due to too much weight.
You can use a really light pack or bag and use your gear to give it some confort, stability and support. Use your sleeping bag and sleeping pad to give the pack support and cushioning for your back. In other words, line the bag with them and stuff everything else down inside them. After all, you only use them at night, so during the day they will be serving double duty and serving you well. In fact with a rope, you could just use a big sil nylon bag (waterprooof), load it all up and tie it off, etc.
Shoulder straps and waist belt are important. Don’t want rope cutting into your shoulders or waist. So I’d look for a pack that has good shoulder straps and waist belt as they are important.
You can have a good carrying heavy duty very stable pack and if you load it wrong, it will be awkward to carry, very hard on you physically and a pain in the butt. Ask Bushwhack about this. So the whole thing is to get the load balanced right and tied down nice and snug.
See you out there. :cheers
Maaintain