In general, external frame packs are simply better at redistributing the weight to your hips, and this is CRUCIAL for long-distance hiking. You get much less tired. Also, external frame packs sit farther off of your back than internal frame packs, allowing for much improved ventilation (although some internal frame packs have improved in this regard). External-frame packs also tend to be larger, and they're obviously easier to strap tents and the like to. Use bungee cords or nylon straps for strapping things like sleeping bags and sleeping pads and tents to the frame. Also, access to the interior of external frame packs tends to be easier, with wide-opening mouths, and external frame packs tend to have many external pockets to help organize.
Internal frame packs are better for situations in which you'll do more active work, i.e. rock climbing. They fit close to your body and thus their weight is easier to control. They also tend to be smaller physically. Some have complex systems of compression straps to make your load physically smaller while still carrying the same amount.
All of that said, if you're not going to be doing any serious hiking (like over 5 miles per day over mountainous terrain), it probably doesn't matter and you should go with the best price versus size and features (IMPORTANT are things like strap and belt quality/thickness -- a 2" hip belt will have you hurting in no time! and curved hip belts fit much better around the hips; other 'features' include number and size of pockets, and external water bottle pockets or internal pouches for things like Platypus-type hydration systems).
As far as size goes, are you going to be carrying everything you need for the whole trip, or will you be able to pick up or buy food/water/etc. along the way? Also, what kind of food will you be taking -- i.e. 'normal' food or specially-designed hiker foods packages or MREs that fit in a smaller space? That will have a large impact on the cubic inches necessary. How many people are going? Will they also have packs? What kind of tent(s)? How big is your sleeping bag, and does it have a good compression stuff-sack (this is a very good thing to have BTW -- your sleeping bag should BARELY fit in the stuff sack)?
Sorry for all the questions, but these all have an effect on what pack you should get

I would recommend visiting a local outdoor shop and talking with the people there and looking up-close at various models. How much were you thinking of spending?