The Sidekick mounts on the side of the chair for what you're actively using (spray bottles, polishers, microfibers, a cold drink), while the Tool Tray bolts underneath for what you've already used or what might fall (loose bolts, dropped sockets, shop rags, small parts).
They're built to work together, not against each other. Detailers and fabricators who run the Complete Package (Robust Steel Max + Tool Tray + Sidekick) consistently say the two-accessory combo turns the chair into a real mobile workstation. Sidekick at hand for the polisher you're using right now, Tool Tray below for the spent microfibers and parts. If you can only pick one, the Tool Tray is the accessory most buyers later say they wish they'd added from day one.