A HUGE advantage of the tri-cabin configuration in a smaller GB-- 36 or 42-- is that if one has guests on board for a cruise, each couple has their own stateroom and head with the neutral territory of the main cabin in between. This means that someone or a couple can get up, make coffee, go for a walk, sit and read, whatever, without disturbing the people in the other cabin.
When we bought our old GB36 we did not anticipate ever taking any other people with us on the boat. Since that time, people I used to work with in television in Hawaii have moved to our area and we have taken them as well as good friends from France on a number of cruises into BC. So we have really come to appreciate the advantages of the tri-cabin layout.
BTW, we use the aft cabin as our main stateroom. The forward cabin when it's not being used by guests is our shop and storage area.
All that said, we feel that for a cruising couple the Europa GBs are the better bet. The aft deck can be enclosed with transparent curtains and even heated. So one can be outside but not be outside.
While we are not going for anything even resembling a GB in our next boat, were we in the market again for a slow cruising boat, we would very strongly consider a GB Europa rather than the tri-cabin for the kind of cruising we anticipate doing from here on in.