I normally carry 3 liters of water, no matter where I hike AND I often end up stealing water from my husband... so keep that in mind. (Putting in gateraid or similar does reduce my water intake, but I normally just drink a lot of water even sitting still - nothing wrong with me, just.. odd :D)

I don't like stopping to refill. So I'd rather keep the weight and not have to stop once or twice to filter (and I'm anal retentive about that too - I don't care who says the water is safe - I don't want to be sick!) I also like having enough water to cook with when I stop to camp.

Pack has to fit the gear, if your stuff is light, but takes up a ton of space, then you will need the bigger pack.

Have fun on your trip!

Just my 2 cents - YMMV