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Not sure about your age and physical condition/ability.... IMO, a backpack style is the better option if you're going to be walking around a lot all day. It will better distribute the weight, whereas a shoulder bag will eventually become uncomfortable, unless you frequently switch shoulders. A sling is kind of between the two, but again, with a lot of gear it will be pulling down on only one shoulder, but won't allow you to switch shoulders like a regular shoulder bag would.
The Fastpack looks reasonable. If you're concerned about water resistance, and if it doesn't have a rain cover, then I would suggest you just bring along a bunch of larger plastic bags, or garbage bags. Sure, it won't look as slick, but it will do the job and will actually be 100% waterproof. The rain covers are often woven nylon, which after a while could let some water through if not properly waterproofed. A plastic bag would also allow you to place the entire bag down on wet ground if you needed to.