I believe the quality of RGB and SCART images is identical. They both come from the 23-pin video port on the Amiga. It's just the connector shape on the other end that's different.
As to your switchbox plan, I'm not sure that's a good idea. These switchboxes are meant for serial ports, not video. There might even be a risk of damaging the Amiga(s). (I'll let others provide additional input on that.) I've used video switchboxes for VGA connections, but those were always plugged into video cards, not the Amiga RGB port. You'll definitely get signal degradation with a switchbox.
Another option would be to plug the A600 into the composite video port and the A500 into the D-SUB port. The signal quality on composite video isn't great, but at least the 500 would be at RGB quality. As I recall, you've also got a C64 in the mix. You could plug it and the A600 into an A/V switchbox (the 2 C64 video cables would need to be merged/converted to composite first). That would also allow you to get the audio of all 3 machines coming through the monitor.