Not to discount online shopping, but you cant touch it, feel it, wear it, or try it online. This is the type of item, that is not a one size fits all type of item. In person, you can bring all the items you would be carrying and see how everything fits in.
As to which one to get, which do you like the best, which one would you use most, where are you going, what time of year, what you plan to carry, and whether or not you have any shoulder/back problems. The time of year, you might be asking why that matters, with any backpack or waistpack, you have a heat issue in the hotter months. Will it be comfortable for you?
I am lucky that I live within driving distance of Campmor, and they carry a pretty extensive selection of Pacsafe. Items I have liked online, I disliked in person, and vice versa.