John - the panniers do NOT differ in size (salesman!) but the handle recess will hold a very small amount if you are caught out in a monsoon - however with the constant movement of the bike being cranked over from left to right etc it will all disappear. I have never had water in my panniers.
Do not know where you are based in UK but I buy my ducs from Phil Worth at Italia Classics in Lincoln. I am sure he'll let you play with a demo '04 model and he usually has 2nd hand models for sale.
One thing I forgot to mention on my last post was the suspension. Being fully adjustable it does take some setting-up to get just right. You can do it yourself of course, but it is a black art in my humble opinion, but when it's spot on the bikes a dream
Well good luck in the search, but the ST4s is a great touring bike in sporting style