I think people are too quick to answer, because it depends on number of hours in the water on a given day, as well as whether or not you are diving one day or several days in a row. Also body type ( how much natural insulation do you have?) impacts the answer. On a liveabord with 5 hours a day under water for 5 or 6 days in a row, I wear a full body 3 or 5 mil even in the warmest water. Water less than 98 degrees F will draw heat from your body. On the other hand, a couple dives on one day in 80 degree F water is a swimsuit only day for me. Its all about comfort , and you should wear what keeps you warm. Have you notice many local DM's in places like Cozumel or other Riveria Maya locales wera a 7 mil full body with hood? They get cold. They wear what keeps them comfortable. So should you. If I was to buy one exposure suit for the most varied use, I'd go with a full body 5 mil, because we go on dive trips just to dive and like to get in a bunch of underwater time.
DivemasterDennis