If it fits, it's better to put it inside. That way it stays dry and is protected. If you put down your pack on rocks or small limbs, it can poke holes in the tent cover or sleeping bag cover if they are strapped to the outside. (In some conditions, a wet sleeping bag can be a safety issue.) The more tired I get, the harder my pack seems to hit the ground. Odd, huh?
Also, it's more comfy if you have the weight of the pack close to your back. When you strap heavy stuff on the outside, it moves the center of gravity away from you, which pulls harder on the shoulder straps.
