Like people have already said, 195/50 gives you a wider choice of tyres at a lower price. I went for that size on my Valver.
Having said that, Ive got some antique 185/55 Michelin Pilots on the Willy at the moment and I really like them! I cant tell how much is to do with the fatc that the suspension isnt lowered like the Valver, but its a very good setup as standard IMO.
Having the extra spring and give in the tyre is perfect for twisty little roads and over poor surfaces - you will get more grip from a tyre like this on a rutted road and be able to put the power down better. Granted, I lose grip and poise in very fast corners (A-roads etc) but I have most of my fun on the smaller roads.
But will you want to pay £30 more per tyre? If they were the same price then it would be a fairer debate - one of the main reasons Ill be sticking is for originality, but youre probably not so fussed.