I prefer my newer SpacePilotPro over the older SpacePilot and the SpaceMouse has a similar design to my Pro, therefore I would probably suggest the Mouse.
Im getting comfy with my new SpaceMouse Pro and would definetely recommend it. I've tried the others more or less as well. The virtual NumPad is just fantastic, leaving the keyboard for typing letters only most of the time! The buttons are a little rougher than the more touchy explorer otherwise it is very much the same and as you say mostly ergonomic difference.