As others here have said, it's not exactly the same to add layers rather than getting a warmer/thicker suit, but if layering around your 3/2 keeps you warm and isn't constricting, great, enjoy your surfing. But a decent 4/3 would likely provide total warmth (you might even get too hot on some days) and better comfort/flexibility than layering.
Here's the real point I want to make: If you want to buy a 4/3, be aware that there are three basic types of suit constructions intended to run the spectrum from from more-flexible-but-not-as-warm to warmest-but-(slightly)-less-flexible. This means that NOT all 4/3 suits are equally warm - by design. On the flexible end are suits that are just a layer of 3/2 or 4/3 (or whatever mm's) neoprene, period: they're great for people who don't get cold easily, or who are competing and want the most flexible suit, but they're not cozy. In the middle are suits that are neoprene with an added lining of fuzzy insulating material inside next to your skin; these are warmer but still quite flexible. Warmest are suits that are both lined inside and have that sticky rubber panel on the outside chest and back; the rubber panels block the windchill and possibly heat up from the sun better. (there are also some other variations, like rubber outside, no fuzzy stuff inside.)
Since you have, ahem, grownup circulation, and since you're longboarder, and sit up high on your board out of the water, you're exposed fully to the wind, so if it's often windy where you surf you'll probably be happiest in the suits with rubber panels outside and the lining inside. If you think of suits this way, in terms of the construction, it's easier to choose between brands and models.
Last edited by Bayztreet; Nov 5, 2012 at 12:40 AM.