Here we go again. Headsets, Sunglasses and flightbags are the 3 major issues that have polarised opinions.
The only comment I'll make is that the stuff I carry is changing pretty quickly. I'm currently thinking about rearranging my bag or getting a new one. 5 years ago, I carried lots of paper. Now I carry almost none. The iPad, EFB and chargers are becoming the main focus.
Think about what you need to carry, put it in a pile on the table and think about the shape of bag that works best. Include everything you need. My flight bag includes panadiene, sunblock, coffee bags, spare batteries, sick bags, spare keys, hex keys for radio trays, screwdrivers, leatherman, torches (3), EPIRB, Pulse oximeter, Co detector, fuel carnet's, Garmin update cards, USB cables, etc.
Periodically I go through and and remove things I haven't used in a while.
I like to have the bag under my knees so it doesn't bother passengers. So I want it to sit upright on its own and to be able to reach down to it in flight. It has a shoulder strap so I can sling it over my shoulder leaving my hands free to carry anything else that I or passengers need. A lot of guys like backpacks for this reason, but I'm too old for the backpack image.