Matt is right on...find out how much you carry and then find the pack that carries it in the most comfortable way to you. I personally went ahead and spent the $$ for a McHale custom pack. Most comfortable and best made pack I have seen and used. If you ask (and with a deposit) you can usually demo one prior to buying. Worth checking out http://www.mchalepacks.com/index.htm
If you want the full spectra pack like the Kelty, it is in the same ballpark as far as cost goes...