While we're at it, an external frame has the advantage of being able to carry larger, bulky items that won't fit in the pack - learned that on a surveying expedition. Military may look 'Rambo' but it will be cheap - if the chips are down who cares anyway. A cheaper climbing / hiking sac will still perform in a short term emergency. In the end its up to you and your budget. I use all of the above as the need takes me. As an experiment I had this actual sac in the photo made for me:
http://www.dndindustries.com/UKpatrolpack.html