Like they said above, find one that fits you. Everyones shape is different and different brands fit some people great and others not at all. I've seem to have the same frame as the Kelty designers, every Kelty pack I own fits great. This is the one I use for day hiking:
http://www.campmor.com/webapp/commerce/command/ProductDisplay?prrfnbr=16440750&prmenbr=226
It has a lot of features you'd find on more expensive packs. Check out llbean.com or rei.com Both have lots of nice packs at good prices.
What you could do is assemble everything you think you'll be carrying and bring it to a good outdoor store. The better stores won't mind if you load up some packs and test drive them in the store. Or just weigh what you'll be carrying, most outfiters have weighted stuff sacks on hand just for filling up packs.
Another thing to remember if you go caching in the winter, you'll need more room for: extra hats, gloves, fleece, hot chocolate and so on.
Have fun