The size should be the size, that is the reason for having sizes. If your 3/2 fits you well you should be OK. Staying with the same brand will help a lot since usually all of the suits are based on the same model. Jumping between brands can be less consistent.
Not true at all...(well, the size
should be the size, but very often, it's not. All sizes within a brand may be based on the same model, but unless the pattern is adjusted each time the material changes, it doesn't matter. Read on...)
One question you need to know the answer to before ordering your suit- does the manufacturer use a single pattern for a particular size regardless of the thickness of the material, or is the pattern size increased as the material gets thicker. Adjusting the pattern for the material is essential if sizing is to remain consistent, but it is often not done (i.e. one size medium pattern is used regardless if it's for a 1mm or 7mm suit).
I had a couple of 3mm suits that fit great, and ordered a 7mm suit in the same size. Not only was was it constricting, but after a few minutes, it had cut off the circulation in my wrists and ankles. I laid it on top of the 3mm suit, and noticed that both were exactly the same size. Initially, you might think that this makes sense. However, the exterior of the 7mm suit
would have to be larger for the interior of suit (the space that your body occupies) to be the same as the 3mm. A little math (subtracting the difference in thickness of the two suits from the radius of the wrist measurement, and recalculating the area), and I was soon aware that the space that my wrists were passing through in these identically sized suits was actually 26% smaller in the 7mm suit. No wonder it cut off my circulation. With bigger body spaces, like the torso of the suit, you won't notice the difference as much - just a more snug fit.
Still with me?
Though the difference in your suit choice is only 2mm, if the pattern is not adjusted, you'll still notice a difference. If your wrist is approximately 7", an unadjusted pattern will result in reduction of 13.64% (wrist only). If it's smaller, the difference will be greater; it it's larger, the difference will be less. I use the wrist as a guide as it's the smallest opening in the suit, and will be most affected by a lack of pattern adjustment.
If the pattern
is adjusted for the thickness of material (not sure of the odds here), then you should get the size that you already know fits you well. If you think the thickness will make the suit more difficult to put on - it shouldn't. If you start with your feet in the suit, invert the suit, and roll it up your legs (pausing at the knees to make sure that they're in place), you should have no more trouble with a thick suit than a thin one.
I hope that this helps - good luck !