because their marketing material repeatedly claims their neoprene is somehow superior enough that you can go to the next thinner size and be equivalent to competitors thicker size. e.g wetwear 3mm = scubapro 5mm for example. There are detailed videos on the site talking about it and they mention it in person. That's the why of it.
not that I'm saying we hate the suit, just that if you're ordering you should expect regular neoprene and you won't be disappointed.
I agree that their marketing on this is a little aggressive. But, I can say that their suit thickness will be significantly warmer that any comparable super-stretch neoprene, especially if you dive below 3 atm or 66'. They just do not compress nearly as much. I have a Bare Reactive 3mm super-stretch, and, while it is a very well made suit with excellent water sealing, it just gets cold at depth due to compression. I have a Waterproof 3mm with less compressible neoprene, and it is much warmer at depth.
Would my Waterproof "equate" to a Bare 5 mil super stretch? Maybe not quite, but it would be pretty close. If your diving is mainly shallow, then you won't notice as much difference and the stretchy 5 mil might be noticeably warner--but at depth the difference will not be so much.
My wife has a custom Wetwear 3mm. She says it is definitely warmer even than her Waterproof 3 mil. She wears it on manatee dive trips and has been in 68-72 degree water for two hours. While she is cool at the end, she is not shivering, and the people in the 3 mil rental suits are out of the water after an hour.
So, yes, get the Wetwear the same thickness as you would typically get for your diving conditions. It will be warmer than almost all other suits of the same thickness and really warmer if you dive deep. And no, don't go "one thickness less" thinking it will make up the difference just due to the lack of compression.
their Chamber testing was done with Rubatex neoprene, which was the best in the business. Rubatex was very stiff, however, and hard to doff and don. So, like always, there are tradeoffs. It seems Rubatex is not available any more for wetsuit construction, but the NCN that Wetwear uses is about the best you can get now in terms of low compression.