Internal

they tend to be a bit lighter, better fit and you can usually cinch these packs up with compression straps so you gear isn't bouncing all over the place with every step, but internal frame need more care packing

internal also better for carrying larger loads IMO, and better for off trail/cross country travel

but external frame are

easier to pack and usually cheaper, may have better ventilation behind the back
also external frame have more pockets with easier access to gear

you may find that an external frame works fine for you for most trips

my suggestion would be to go to a place like REI and rent packs of either kind on 10 mile or so trips


also I have to add that internal frame packs are the in thing these days, external frame used to be the dominant pack about 20 years ago, but internal frame are the more common pack now, so you will find more variety of internal frame at the better backpacking/camping stores


And I've never had trouble lashing things to the exterior of internal frame, usually you can lash/stow sleeping pads either horizontally under the top pocket, horizontally at bottom ot the pack or vertically to the center outside of the pack

but heavier items should go inside close to your center of balance (closer to you back)

I recently purchased a Kelty Agile 4500 internal frame backpack and I am more than satisfied with it! It's the first time I've been able to easily pack all the gear I planned on taking, and then was able to access all that gear when I needed a particular item.
There is provision for a hydration bladder at the top, but I prefer to carry two one-liter water bottles in the mesh pockets on the side. A collapsible water container is used when I find my last water source of the day. This ensures I have enough water to handle cooking dinner and breakfast and to refill the water bottles the next morning.
I tie my self-inflating mattress and stuff sack containing my shelter's collapsible poles and stakes onto the top of the pack. Sleeping bag and 2-man tent fit into the bottom of the pack easily, as well as spare clothes and other gear, plus a seven-day supply of food.
Lots of compression straps to cinch down the loads and adjust the shoulder, sternum and waist straps to the sizes I need and the frame is bendable to mold it to my back's curvature.