I am a big guy and need a pack for multi-tasks. I sweat profusely so a hydration system would be nice and the only advantage I see to an external frame pack is it is cooler.

I will be buying this pack for a Christmas retreat with my wife, and plan to pack it with the usual stuff. However, I would like to set it up as my paranoid-pack.

I want a lot of space and external pouches for quick access, so please inform as to the advantages of both types.  

External Framed:

Pro's:

1) Better at carrying heavier loads on flat surfaces.
2) better ventilation than internal framed packs
3) can be used to make a stretcher if you ever needed one.

Cons:

1) takes up more space than an internal framed
2) Not as stable on rugged terain, the weight tends to be higher on external framd packs, so when you start tipping over, you usually go over.

Internal framed packs:

Pros:

1) More condensed volume/pack size ratio.
2) More stable on rugged terrain.

Cons:

1) poor ventilation as compared to external framed packs.

I'd say he's just about laid it out for ya.  Think about your terrain (I live in very brushy steep wet country in the Pacific NW).  Will a lot of junk hung on the outside just get snagged?  Does everything want to be inside, and the pack somewhat "streamlined?"  

If you want versatility on a modest budget, an external frame like the Camp Trails Freighter lets you pull the bag off, and lash just about anything against that folding shelf.  

Personal preference counts, along with intended use.  My sister spends more time in the hills than I do, and she likes internal -- also better for ski touring because weight is closer.  But she travels light.

My daughter used the same internal frame "convertible" by Eagle Creek in Search And Rescue, that she did going to China a few years earlier.  I lean toward internal frame or small external (like ALICE) for a bugout bag.  How will you carry your BOB, and does appearance matter?  (Some folks don't want a military-looking pack visible in their vehicles.)  

I have always favored external frame for any significant weight and for the ventilation.  I do NOT like anything hung onto the outside, so I go for a big bag on that frame.  But as I get older, I'm working at avoiding that "significant weight," if at all possible.

It may be that you are talking about two different packs for two different missions.  As with personal arms, it's nice if you can try out several before laying down your hard-earned long green.  

Caveat emptor

Many outdoor outfitters (backpacking shops) have demos of most of their packs and sleeping bags.  rent one or more before you buy then you will know for sure which one is best for you.

Any good outdoors shop will have a demo pack and a 20 pound weight for you to try on and walk around with.  If you can, like stated, rent the pack first.  EMS used to let you rent all your gear, then knock a little off the price if you decided to buy it.

Don't ever buy a pack without first trying it out with some weight in it.  It's like buying shoes online without trying them, except usually a lot more expensive.

I just thought of this, as pertaining to Military packs.  Civilian packs deal with heavier loads by having there Backpacks pack upwwards.  This helps balance out the heavier loads.  This however would not be a good idea in military packs, because from the from it makes you a bigger target, and harder to keep your head down.  They, instead, pack outwards, which is a little less comfortable that civilian packs, but leaves a smaller profile.

You may want to keep that in mind when deciding to buy a Bug Out Bag vs. a Camping bag.