Decide on what you want from the bag:
just a bag that can carry your gear
compartments to organize (bear in mind the bag does not have to come with compartments, you could make them - but consider the balance between saving money on purchase, being able to customize just for you, and the ease/difficulty of modifying
How heavy it will all be and how you will carry it - wheelie bag, back pack straps, should strap, handles
Also how you will transport it - a big bag takes lots of gear but is hard to handle on buses, mopeds etc
What are the other benefits - a solid case can become your seat, work bench etc
Roller tool boxes are great for gear