If you can swing the cost, get a drysuit. It's slightly more work than a wetsuit but oh man it's nice to be warm in cold water.
I personally do not buy into the "special neoprene" that gets sold by some manufacturers. Well, I did (literally) but not anymore. I got my wife a fully custom wetsuit from wetwear.com. It's a perfect fit. They talked us into the 3mm because their special neoprene was as warm as a 5. Nope. It's not. She won't even dive in the Florida springs with it (72f year round). Maybe the brand you are looking at has something that wetwear doesn't.
Dive some rental suits if you can. If you can't, then just go for the 5. Personally, I wouldn't want to be in water much cooler than 70 with less than a 7mm, drysuit, or multiple wetsuits layered.