Open cell suits are the preferred wetsuit for freediving. They are warmer than the same thickness closed cell wetsuits. The slick interior lining almost adheres to the skin of the diver. This keeps a thinner layer of insulated water against your skin. This allows thinner suits to be used in colder water temperatures. Most open cell wetsuits have an integrated hood, which helps keep the diver that much warmer. The hood also helps protect the diver from the underwater environment.
It should be noted that there are different qualities of neoprene. The elasticity and durability are the main competing qualities of neoprene. The more durable the neoprene the less elastic the neoprene tends to be. Elasticity is what helps a wetsuit stay warm. The air bubbles along with the thickness of the neoprene are what help insulate and give elasticity to the suit.
Open cell wetsuits are challenging because the interior of the suits are sticky when they are dry. When the suit is wet they are easy to slide on. Divers need to wet or lubricate the interior of the suit before putting the wetsuit on. Most divers carry a bottle of warm water with conditioner or baby shampoo to put on the interior of the suit, or to wet their forearms and hair before donning their suit. If you don’t lubricate the suit and attempt to put the suit on it can damage the suit, and result in significant discomfort to the diver.
Open cell wetsuits can deteriorate more rapidly when used for scuba diving. By compressing the suit for extended periods of time it permanently compresses the air bubbles in the rubber. They still make great scuba suits, they just wear out faster than closed cell wetsuits might.
Closed Cell wetsuits area easier to put on. The sad truth is that no wetsuit is truly easy to put on. Closed cell suits usually have a fabric coating on the interior to allow them to be put on easier than closed cell suits. Many closed cell suits do not have an integrated hood, which some divers prefer. An exception to this is the Neptonics Quantum Stealth 1mm, which is designed just like all the other Quantum Stealth suits, but constructed of neoprene foam. This suits primary purpose is less for keeping warm, but for protection from the surrounding environment. Closed cell wetsuits tend to be slightly more durable than your average open cell wetsuit because they are a denser neoprene.
The draw backs to closed cell wetsuits are that they are not as warm and not as flexible as open cell wetsuits. This is because the neoprene is denser than most open cell wetsuits. The fewer air bubbles stored in the rubber the less it is able to insulate the diver. Flexibility is very important to freediving efficiently, but may be less critical when scuba diving.
there are no hard and fast rules on how thick your suit should be. Every diver has their own tolerance for the cold. As a result, In order to dive in a location year round you may need to have a couple suit thicknesses to be ready whenever the opportunity presents itself to dive. Warmer conditions you may only want to use a Rash Guard to prevent abrasions form aquatic life. In colder conditions you may need a 3mm, 5mm, 7mm, or in extreme conditions even a 9mm wetsuit. A good rule of thumb is a 10 degree farenheight temperature for wetsuit thickness.
Again, this is not a hard and fast rule. Every diver will handle cold or heat differently and may want a different thickness suit for a given temperature.