The next step up appears to be a choice of 3/2mm and 3mm full suits. Any thoughts or recommendations on the differences between a 3/2mm and a full 3mm suit?
I am skeptical that it makes any difference whatsoever.
I think what makes MUCH more difference is:
- fit
- stitching
- seals
- lining
The best 3mm with glued-and-blind-stitched seams, seals all around, and a nice lining is not going to be as warm as a crappy 3/2 if the 3mm fits poorly and the 3/2 fits really well. That is possibly a slight exaggeration, to make the point that fit is the most important criteria (in my opinion, anyway).
GBS seams are definitely way better than flatlock stitching.
Some suits have nothing to really seal any of the openings. Some have seals at some of the openings but not all. The best suits have some kind of seal at the ankles and wrists, the neck opening, and something behind the main zipper.
Some suits have no lining. I.e. they're just nylon-covered neoprene on the inside, just like on the outside. Some have a fleece-like lining on the inside. Of those, some have the lining only in the chest or torso. Others have a lining that covers nearly all of the inside of the suit. I'm not sure how much difference the lining makes to warmth but my personal anecdotally-based opinion is that it does help.
I have been diving a ScubaPro EverFlex 3/2 for over 3 years. I have found it to be adequately warm even in 70F water. It has glideskin (aka smoothskin) seals at the wrists and ankles, and the inside of the neck, and has a nice pad behind the main zipper to help seal there, too. It has GBS seams. The newer models have a DiamondSpan lining that covers most of the inside of the suit. I have some boots with the DiamondSpan lining and it does seem to make them extra warm. The current EverFlex 3/2s also have a short zipper at the front of the neck, so you can unzip a bit if you're too warm in the water. If it fits a given person well, I expect it would just as warm as any 3mm suit out there.
I'm not trying to push ScubaPro. I'm just offering an example to illustrate that there ARE good quality, warm 3 or 3/2 suits out there. Other companies make suits that seem to be totally comparable. But, if you want a 3 or 3/2 suit that gives maximum warmth for its thickness, you will pay more.