If duelingbook is anything like duelingnetwork (which by looking at the login page, it's certainly giving me the DN vibes), where you have to manually play your cards, I think it should be avoided - but I'll check it out.

Also I vouche for YGOPro. It's quite nice; very smooth and has always been free. They keep their banlists updated. I'm sure there is a spectator system implemented although I've personally never used it.