This is totally a matter of preference. After starting with a Kelty external frame pack years ago, I have been using an internal frame pack since 1988. I like the freedom of movement especially when bushwhacking in tight places & packing into high camps above timberline. This type of pack actually feels like an extension of my back, affording me better balance without having to make corrections as a result of being side or top heavy. I also like the way all my gear fits inside giving it a cleaner look & nothing to snag tree branches, etc. I haven't had to use external accessory straps since then.

Re: Comfort - Internal vs. External Internal frames: sit closer to your body and generally are taller and slimmer = not likely to get caught on branches = easier to move around. External frames: sit on an frame so there is some separation between the pack and your back = your back won't get as hot and sweaty as with an internal frame pack. External frames: for their capacity are usually much lighter than an internal frame pack with similar capacity (sometimes as much as a pound) = lighter pack is going to be more comfortable over longer distances. Note: An internal frame with back venting features (i.e. mesh) are going to be more ultralight (= less durable) and the capacity will be less than an external frame with comparable weight.

It really depends on the type of load you are carrying and where you are carrying it. External frame packs are load warriors: packs designed to carry large loads for long distances over relatively well established trails without a lot of low overhanging branches or narrow passages. An internal frame pack is ideally suited for trips where balance is critical or where narrow spaces will be encountered.

I have found internal ones to be more comfortable.