Before byuing bag for my equipment I made quite extensive research. At first I had to decide what type of bag I want. Backpacks are good for carrying gear, however, they aare inconvenient when you need to have quick access to it. Accordingly, I decided to buy messenger type bag.
Secondly, I looked what is optimum equipment I will be carrying and actually using. It is not wise to put all your equipment in one bag and carry all stuff if you do not have intentions to use some of it (e.g. flash, or macro lens).
Thirdly, I was looking at quality of bags as I do not want to constantly buy bags and spend my money.
Based on that, I chose ThinkTank Retrospective 30 and I am very happy with my choice. I hope that this logic might be useful to you as well.