Small to medium internals often have side pockets that work well for water bottles but lack other external storage. Most external packs have side and back pockets that are nice for storing small, readily accessed gear. Most people start with an external but find need for an internal for specific activities. I like an external for general backpacking and an internal for search and rescue. Make sure your pack fits well and is padded in the waist, back and shoulders.
Good packs have torso length and shoulder strap adjustments.
Try it on loaded with weight before you buy. All of us have some experience with a pack. When buying a new one consider what has worked for you and what you want in a pack. Remember, it is your most important item in terms of comfort so don't buy a cheap one. A pack with a volume of 2000 to 4000 cu.