External frame packs allow you to pack a load higher, keeping it more in line with your center of gravity. You can’t do this with an internal frame; they require a larger, heavier load has to be worn further from your center of gravity. Obviously anything further from your center of gravity is harder to carry.
Internal frame packs are lighter, and are designed for smaller loads. Because the load will be low and close, they are more ergonomic over rough terrain. When everyone was using externals in the 90’s the only internal frame packs you could find where “technical bags,” made for rock climbing.
So which is better depends on the size and weight of your crap as well as what your doing. If it’s small and light you don’t really benefit much from an external frame pack. If you’re going over moderate or difficult terrain an external frame might try to toss you off balance.