I've had all three and I reckon that each has it's merits, depending on the bike.
My belter was a GPz305 back in the eighties on which I despatched. A wonderfully efficient drive system, it hardly ever needed adjusting and the only maintenance required was a dusting of talcom powder every now and then. I lost heart with it when it decided to take on a stone and lost.
With a chain, if it snaps, you can usually get a replacement fairly quickly and feed it through. I think I was waiting about ten days for a new belt from the big K. When I PX'd for the Gpz550 I immediately remembered why I hated chains!
I've had three Guzzis and I thought 'ah, this is more like it' , as you do. Despite rigourous attention to serving I managed to chew up the rear drive box of my Monza which in fairness was quite easy to repair. So ever after, I've not felt 100% comfortable with shafts either...and of course, you'll be lucky to find a replacement on the shelf at your local bike shop.
Times have moved on haven't they? O rings, Scottoilers and the like make it far more sensible than it was to lash on that ancient oily device that the Tiger boasts.