? As Rusty is saying, start with your size, get a pack that will fit you, as you'll be able to carry more and be less fatigued with a properly fitting pack. Figure out what you're carrying compared to who you're going with - and who is carrying what.
Where are you going, how far under foot power, what type of terrain and weather are you expecting, how many other folks are going? A T3i with 2 lenses are better added to a traditional backpack than a special photo bag purchase. Check out rental options on both types of bags, since you may not need the bag for another year or two and what you carry will change.