+1 for everything Glenn said! I personally cannot use Osprey; their hip belts and my hips consistently disagree, with much pain to the latter. But lots of folks do well with those packs. I personally prefer a smaller and lighter pack, but my gear is pretty light and compact.

The most important part, though, is not the brand, or the features, but fit, fit, and fit--pack has to fit you, fit your gear and, most of all, be comfortable for you with your specific gear in it. Pack fit is almost as individual as shoe fit! That's why nearly everyone recommends purchasing your other gear first, your pack last.