Not very important, in my opinion, since even the thickest of these suits offers very little insulation. Claimed neoprene thicknesses range from 0.5mm to 1.5mm, but actual thicknesses are much closer together. The 1.5mm Bare Sportswear suit seems to actually have no more than 1mm of neoprene, for example. To this, add about 0.5mm for each layer of nylon. As much as possible, I try to remove the subjectivity from gear tests and rely on what can be measured.
Every diver has a different shape.
Every diver has different thermal needs. So here's what I did. Water temperature was 80 degrees. I wore all the suits enough to know which ones fit me, which didn't, and how much that mattered. Different ones may fit you best, of course.