From Boy Scouts until this summer I always used an external frame pack. I bought a Jan Sport D3 in the 80's and used it until it broke a few years ago. JS has a lifetime warranty on there products so I mailed it into them to get fixed. They did not have the parts and offered me a choice of 3 different packs. I chose the retro module of the D2 they put out for few years. You have to Google it to get a sense of how big the pack is, but about 6 lbs. My first trip to the high Sierras this summer got me thinking about weight, so I bought ULA Catalyst at about 2.5 lbs. Though the ULA pack carried vey well and my weigh at the start for a week was 33 pounds, I found it very different then the JS pack in a few ways.
First most internals are top loaded. So you have to figure out how to pack them. Also you may not have outside pockets and they don't stand up very well on the ground.
So here's the trade off I see on with the JS pack I have. Frist he JS transfers 100% of the weight to your legs, unlike a internal frame. So you can carry more weight as nothing much is on your shoulders. Also you expend less energy packing it or unpacking it on the trail if you need something deep in the pack. Also you don't have to baby it putting it on and also taking it off. You also have the option of putting the bear canister on top and packing your food directly into the pack. You then use the canister only at night at camp. Plus many sites have bear boxes so you may not even have to transfer the food some nights.
I believe you could get 10 days worth of food and all gear in the JS pack and be around 40 pounds. That's assuming you have the lightest gear possible and have shaved off unneeded items.
For trips up to 7 days the internal packs probably serve you the best for total weight. But if your doing the JMT and your last drop is at the Muir Ranch, you have 100 plus miles to go. Unless you want to go out Onion Valley for a day or pay big bucks to get something packed in. So on a long through hike where resupplies are no issue, internals work great. But if your looking for 10-14 days without getting food, the externals will serve you better.