I'm not a great fan of Ryobi anything but the 3 x 21 works to perfection, but get the sanding frame. It will reduce of the likely possibility of ripples with aggressive belts. The sanding frame is not cheap but get it anyway.
The frame is a royal PIA installing as there are springs that want to fly as soon as they get the chance of freedom. Still the frame is a good idea. I does its job and makes your work easier.
The Ryobi can be turned upside down and used for quick sanding without building a separate box to house the sander. I use it that way often to rough out a shapes like for hammer handles.
Also, I don't know about the innards, but Bosch has a sander that is a twin to the Ryobi. The Ryobi is less expensive though. Not the heavy duty sander that is hard to control, but you won't be disappointed unless you really need the big guns.