If you use your suit in salt water you might consider getting a lycra rashguard. Salt residue can build up on the inside of the suit and rub against your skin. The thicker the suit, the 'better' the suit is able to insulate. If the suit is too tight, you won't be able to move or breathe. If the suit is too loose, too much water will get inside, adding extra weight.
Metal zippers are better than plastic.
Suit with zippers in the back are supposed to be stiffer and less flexible than shoulder-zip suits. They may keep the water out better, but they are hard to get in and out of.