When it comes to backpacks, since 2007 my preference has been for frameless models.
The reasons are as follows:
1. Simplicity: Frameless packs generally have simpler designs. Less zippers, less compartments, less straps, less that can break, rip or go wrong.
2. Fit: With frameless backpacks it is the load itself which provides the structure.
With your sleeping mat acting as a makeshift frame sheet, ideally a frameless pack should mould to the contours of your back. In order for that to happen, particular attention must be paid to the manner in which your pack is filled.
A heavy load packed indiscriminately into a frameless model, will be a lot more uncomfortable than it would be in a more forgiving internal frame backpack.
3. Weight: Frameless packs are lighter, but not at the cost of durability. The one caveat to this point is if you consistently overload them, in which case they will eventually breakdown as a result of excessive strain.
Case in point are the two MLD Burn packs that I have owned since 2009. Each pack has lasted at least 9,000 trail miles (14,484 km)…………the second one is still going strong.
4. Size: It’s a funny thing, but people who buy a pack with a large carrying capacity (e.g. 65 litres or more), generally find ways to fill it. You have all that space, it would be a shame to see it go to waste, no?
An advantage of frameless packs is that they are generally small. They are not made to carry loads over 11 kg (24.3 lbs) for extended periods of time. This limitation forces the hiker to pare down their backpacking kit.
With a frameless pack the focus is on carrying the necessities, with superfluous luxuries either left at home or kept to a bare minimum.
5. Cleanliness: Easier to wash and keep clean. I tend to throw mine in a front-loading washing machine every few thousand miles or so.
6. Cost: Generally speaking, frameless packs are cheaper.
NOTE: If your base pack weight (i.e. total weight not including perishables) is more than 6 kg (13.2 lb), you are probably better off with an internal frame pack.