Sorry for the late reply! I'm only going to Europe for a month so not nearly as long as you, but like you said we're limited on the boat. I'm taking a backpack because I know that the streets in Europe are terrible for anything with wheels. I also would rather the mobility of a backpack rather than a suitcase because I don't want to look vulnerable if I can't move it around properly.
This is the one I'm looking to buy. I'm going on a bus tour before the sail and they gave us specific dimensions and this one is a tiny bit bigger than the dimensions. I read somewhere that they don't care too much about dimension, but more about the weight. I'm looking at packing around 12-15kgs so I can fill up when I'm there if I want to.