What's your budget? A backpack is like a plate carrier. If you buy a knock off or poor quality one, toss 30-40lbs of plates, mags, and hydro into it, and then start running/crawling/dragging, and it'll come apart fast.
...It's also similar in that it's really hard to find anything in cadpat.
Also, what size are you looking to get?
If you have a backpack already (for school or whatever), I would recommend trying to pack what you need, into it. Then see if it's large enough, if there are enough pockets, if you think you'd want your hydro pack in the backpack or in your chest rig (remember, you drop your backpack, and you drop your water), if X is missing, etc. Then put it on over your chest rig (or however you would need to carry it), and see how awkward it is, and/or if having a bigger bag would impact you significantly.