I've got a similar problem in the height/weight category. I'm 5'8" and 230. According to the sizing charts I needed to be 6'2" (in my wife's dreams maybe). Tried on a 2XL 7mm AquaLung AquaFlex at my local dive shop and was surprised how well it fit. Went home to tell the wife what I wanted for my birthday and just happened to look on ebay. Found one of that size practically brand new for a fraction of the cost.
If you're buying off the rack, buy according to your weight and DEFINITELY get one made out of the Hyper/Super/Maxi stretch material. Try it on to make sure it fits your problem area (belly). The legs might be a little long, the arms too, but you can pull it up or have it shortened at an alterations shop that knows how to work with neoprene. Off the rack won't fit perfectly, but will work nicely. If you can find what you want on a deal someplace, even better.