I prefer internal due to the terrain I generally hike on. I did a write up about the differences between them. Let me see if I can find it.
ETA: Found it. This is just my opinion based on my exerience and from what I've pulled off of technical sites.
Pro's and cons of internal vs external.
Internal: Pro's. Keep stuff drier if you don't have a pack cover, organization can be easier. In my experiance, they're more comfortable. If packed correctly, they keep your center of gravity closer to what it'd be without a pack. This is good for alpine type climbing, mountaineering, and rougher hiking. These packs are better suited to rougher trails where balance is needed than an external.
Con's: Usually more expensive, once it's full, adding stuff to the outside can really start screwing up balance. Harder to carry a very heavy load.
External: Pro's. Cheaper, very simple, easier to fix int he field. Great for hiking long distances on flater ground. They also carry very heavy loads better. Can strap tons of crap to it.
Cons: Center of gravity is higher, which screws up your balance. If you're going to be doing any climbing, or hiking on rougher terrain, you'll notice it a lot more than with an internal pack. Can be uncomfortable, but newer ones should be better. My wife says they're "ugly".
Because of where I live, and the type of hiking/camping that I generaly do, I only have internal frame packs. I have a kelty, Eureka, The North Face, Lowe Alpine (actually my wife's), and 2 Mountainsmith packs. All of them have worked well. My Mountainsmith's are probably my most comfortable.
Campmor.com, sierratradingpost.com, rei-outlet (from time to time), and ebay is where I have bought some of mine.