A simple, closed-cell suit like a surfing wetsuit works above the surface where you have heat from the sun and little pressure, but when you get below the surface, it can get stiff and cold. An open cell suit will keep you much warmer and more flexible, whether you're freediving or using scuba tanks.
I've never actually owned an open-cell diving suit — I use a surfing suit to dive, which I assure you is less than ideal — so I called on a lifeline: an old friend who spends his workdays and sometimes his nights underwater in the marrow-chilling depths of New Zealand's Marlborough Sounds. If anyone has earned the authority to deem a wetsuit good or bad, we figure it might be a commercial diver, after all.
The array of both open cell and closed cell diving suits in the locker where he works is almost exclusively with Beuchat and Cressi wetsuits, and while many of the members of the dive team do wear closed cell suits to work, they don't last as long — maybe that's intended. Open cell suits are snug, and almost suction-cup your skin, which is extremely efficient for keeping you warm, but makes them very difficult to get on and off.
When we would go spearfishing together — I in my 5/4-millimeter closed-cell surfing wetsuit, he in his 7-millimeter open-cell diving suit — I'd be in and out of my suit in half the time it took him to roll his on and off. But, by the same token, he could still feel his hands and feet after an hour of diving. Meanwhile, my lips would be turning blue.
Bottom line: If you're going to be in even moderately cold water, save yourself the agony of freezing and put up with the nuisance of stretching into a skin-tight open cell suit.
If you've never worn or owned a diving wetsuit before, you'll probably want to go to the local dive shop and have the pros sort you out, or at the very least fit you.
When picking out a diving suit, color, or rather pattern, is a consideration that goes beyond aesthetics. If an experience with wildlife is what you're after (even if you're not in search of dinner), then a camouflage suit is probably a good idea, simply because you won't startle as many creatures as quickly as you would with a black suit, or one of any color, really.
Also, note that camouflage is relative: If you're going to be in open water, you'll want a rhapsody in blue, and if you're going to be in kelp, coral, or rocks, you probably want to look for a more greenish-brown pattern.
The main downfall of many closed-cell suits is that they are made of or coated with a softer, more delicate rubber-like neoprene skin which, while it keeps you warmer and leaves you agiler in the pressured depths, is highly prone to tearing.
Also, always make sure your wetsuit is wet when you're pulling it on, and follow instructions for care and maintenance like these, from Aqua Lung. Never leave any wetsuit in the sun but especially not a suit with skin material, which will melt and stick to itself, a tragedy not covered by any warranty far as I'm aware.
Aqua Lung, Beuchat, Cressi, and Mares are companies that have all been around since recreational diving has, more or less, and they all have similarly long legacies and popular standing with commercial and recreational divers alike.
Pros: Tighter-fitting, more watertight, keeps you warmer, less constricting
Cons: Can be more expensive, much more delicate, difficult to don and doff