Choose the swimming wetsuit based on the swimming style that you are going to practice. A thin suit (ca. 3mm) is good for breaststroke since thinner neoprene doesn't float or lift feet too close to the water surface, which would make the frog kick harder. A thin suit is also good for freestyle swimming. -> If you are going to do breaststroke or breaststroke and freestyle, choose a thin suit.
A thicker suit (ca. 5 mm) is the best for freestyle, since it floats better and make your swimming position better, or lifts your feet. The breaststroke kick becomes more difficult in thicker suit, since your feet are floating too high. -> If your style only is freestyle, choose a thick suit.