personally, I would not buy a wetsuit online, that you have not tried on before. they all fit differently, regardless of what the sizing chart says. some people have a longer torso, or longer legs, but may be the same overall size, and the suit fits both completely different.
with that said, an undersized suit is extremely uncomfortable as far as being able to breath goes. an oversized suit can cause rashes, and bring in water through a gap in the neck, etc, etc.
good luck with your purchase.