As a general rule, if your total pack weight is going to be consistently over 25 pounds, a pack with a stiff frame is a good idea. It will weigh more than a frameless pack, but the increased comfort provided by the effective transfer of load bearing to the hips will be worth it. Under 25 lbs (especially under 20), a frameless pack is plenty comfortable, and the weight savings can give you a considerable speed and mileage advantage will be worth it.
With this in mind, and expecting to have a base weight (not counting consumables and what I'm wearing as I hike) in the 7-10 lbs range during the hike, I will be using frameless ultralight packs for most of the journey, and a semi-stiff large capacity pack for the Sierra Nevada, where I will be most loaded down with extra gear (bear canister, ice axe, crampons, warm jacket) and 8 or 9 days of food.
I would like to try out all three of my packs on the PCT (evidently, I have not always been on as strict a budget as I am now!). I expect to start with the Mountain Laurel Designs (MLD) Zip pack (2008 model). It weighs about 300 grams (10.5 oz) and has about a 40 liter capacity. During the first 1-2 weeks, I expect to have a slightly depressed appetite, and there will be plenty of volume in the pack for the 3-5 days of food I'll need between resupply towns in this section.
At some point as early as Big Bear I plan to trade my Zip for my Zpacks Blast 36 pack, which weighs 200 grams (7 oz) and has a capacity of 50-55 liters. This pack is a bit worn already, having seen 50 days of backpacking, but I expect it to hold up for another 50 to 100 days. It has convenient storage bags on the hip belt and shoulder straps, where I put all my little stuff and carry maps and camera. I think I will appreciate the extra volume when my trail appetite comes into its own and I'm wolfing down 5000-6000 calories a day. I'll be able to pack high-volume food such as bread, spiral pasta, etc.
For the High Sierra starting at Kennedy Meadows (700 miles into the trek), I'll use my Golite Pinnacle pack. It has a semi-stiff back and seems to carry up to 30 or 35 lbs in relative comfort. It weighs 700 grams (25 oz) and has a volume of 70 liters. It's also made of a tougher fabric than the other two packs. I'll need the added volume for the mandatory bear canister and for the warm jacket I plan to take. I expect to use a plastic pack liner every day to guard against getting food and gear wet during river crossings. As soon as I can get rid of the extra gear (probably Sierra City, nearly 500 miles later), I'll trade in the Pinnacle for the Blast, which I'll probably use for the remainder of the hike.