I am 5'10" 175 lbs medium built. I went with the medium size and I am glad I did so far. I was told to go large, but medium fits me really well. I tried with 2 layers of thick polyester coats and thick layered pants, I could move easily without any problems at all.
I personally would recommend getting it from a place that could exchange it. I ordered it through Clavey.com and we had to exchange my wife's size (large to medium), they didn't give us any grief at all.
If I were you, I would order it from REI and use their 20% off coupon, you could save $98 dollars off their $485 retail price. Kokatat Tropos SuperNova Dry Suit at REI.com
REI is awesome at taking stuff back and exchanging it too.
A buddy of mine just got back from GC and he said there were two SuperNovas there, one took a swim through Lava. He said the guy was completely dry inside even after swimming such a big rapid.
Hope it works out well. Also do a search on this forum, there are quite a few threads about this same topic (I started it myself