If I were designing the perfect temperate water wetsuit it would be a 5/3. (or 4/2mm for warmer water) 5mm standard neoprene on the torso for warmth and 3mm on the limbs for flexibility and ease in getting it on and off.
It isn't always a matter of how warm you are, but how warm you feel. My arms and legs are far more forgiving of cold than my trunk, and if I feel the slightest bit cool on my stomach I feel cold all over.
BTW- note that thickness alone doesn't translate directly into insulation. Some grades of neoprene resist compression better than others and therefore retain more their insulation value at 60fsw or more. Also, while the wetsuit companies may down play this, I've found that the old fashioned foam neoprene insulates better than hyperstretch of the same thickness.
Ask around about particular suits, or squeeze the neoprene over the torso, and opt for the firmest.
Lastly, fit trumps thickness, an ill fitting wetsuit that allows water to flow through it will always be cold, no matter what the specs are. Make sure it's comfortably snug at neck and wrist seals, and has a long enough zipper to allow you to get it on and off without tearing it apart at the seams.