I commuted for 10 years. Either around the waist, or in a pannier, I carried an old Tamrac camera bag. Usually one half was taken up by a glass peanut butter jar filled with whatever (usually left overs). The rest of lunch was whatever I could fit in the rest of the bag. Maybe supplemented with coke and chips from the vending machine. In the end I chose by what was convenient to carry (size) and the need to use the microwave (glass container).