This is how Huub is describing the difference.
http://www.huubdesign.com/science.html
3MM UPPER BODY MATERIALS AND 5MM LOWER BODY MATERIALS
85% of triathletes come to the sport from a non-swimming background and many suffer from low sinking legs in the water as they swim. Of course the buoyancy of any wetsuit helps bring the legs up higher but never before have wetsuits been designed specifically for these swimmers with a large buoyancy difference between the upper and lower bodies. Until you swim in one of these suits you won’t believe how high your legs can be, they will feel glued to the surface and your potential as a swimmer will be truly unleashed!
As we designed these sophisticated high technology suits, we increasingly moved away from neoprene towards new materials with properties of increased buoyancy and specified levels of stretch.
4MM UPPER AND LOWER BODY MATERIAL THICKNESS
Our 4:4 wetsuits are for the stronger swimmers who already have a great horizontal position and do not need the redistributed buoyancy of a 3:5 wetsuit. You’ll find perfectly distributed buoyancy to maintain the balance, harmony and efficiency of your stroke technique just as it feels in the pool. Like to use a bit of kick as you swim? No problem, these suits will let you do just that with a smooth natural action.
This is how I swim and guess what suit was the best for me?
Yes, for me the 3:5 is a better suit. Do not get me wrong. When I tried the 4:4 it was an amazing suit I had problem understanding that just move some rubber around it would be a difference. But for a swimmer with no swim background, the 3:5 is a kinder suit. It helps you to deal with your issues. To describe it in cross country skiing terms. The 3:5 is a ski that will make a lesser good skier ski better, the 4:4 is equal to skies made for the best. I could feel the difference and I could really feel that my body position was better in the 3:5.
Huub Archimedes is a well made suit. But any user should make sure they take the time to put it on correctly. It is not rocket science, but if you want to get the best out of the suit it has to fit, it is that easy. You would not start running without tying up your shoe laces or start biking without clicking in your shoes. Putting on a wetsuit is the same.