Although I have never backpacked to fish, I have backpacked to cover miles. I have a 3100 cu in Osprey pack that was very reasonable in price.
I can imagine if you will be stuffing waders and tackle into the pack, you will need something with more storage. Its important to be as efficient as possible, so you dont want to go overboard and have too much space. Learning how to load a backpack probably more important than how much space the internal compartment has. Keep that in mind.
Secondly, Osprey is the only pack that I have ever owned that does not fall apart. On top of that,they have a wicked warrenty. There are several models to choose from, so it becomes, largely, a matter of preference.
Whatever you decide to purchase, be sure that you have gone to a reputable outfitter and have been helped by someone who is more than competent. If the sellsperson does not drop a few weighty items into the pack and make you wear it around the store for a little while, then dont buy.
Hope this was a little bit of help.