I'm not sure about the others, but I've owned the Saint James Breton, and found it far too thick for my tastes. It could maybe be worn in the UK during the summer, but in the US I think I would die. Where I live temperatures pushed 102 yesterday. I love the quality and the design, but it's a shirt I can't wear. Now, if this drop were to help me discover a higher quality breton with a thinner material... I'd be all over it.
molognaYeah, I wouldn't describe their carded cotton as summer weight (of course, I live in San Francisco, so summers are like 65 deg F). I think they do best in the Fall, personally.