I've not built the st Roch, so no comment on that, but I am the builder of the 16 inch tug in this months model boats magazine. I can say with certainty the stability is no problem what so ever. Common sense whilst building, no excess weight up top, ballast nice and low and spread the length of the keel. Planning in advance where all the working bit go helps a lot. Just choose where you sail carefully, other modelers don't always notice a small model on the water.
Chas.