I've been diving dry for close to 20 years now.
Neoprene
Neoprene dry suits have their place. If you get one that is fairly snug and wear only a wicking layer underneath, you'll have a very streamlined suit that swims very well - much like a wets suit and with similar weight requirements. Neoprene seals tend to be very durable and last just about forever, plus they provide insulation at the neck and wrists where blood flows close to the surface, keeping you a lot warmer. The neoprene fabric contains the insulation and a 5mm or 7mm drysuit is a lot warmer than a 7mm wet suit, and is comparable to a tri-laminate drysuit with intermediate weight underwear, even at depth. They also do not suffer from the condensation problems that plague a trilam as the insulated "outer" prevents that. A snug suit is an option as the material stretches, so fit is less critical. The downside of this approach is that it limits your really cold water options as you can't stuff much underwear in it. O'neil used to make an absolutely superb 5/7mm neoprene drysuit and it sold new for around 500.00 and Atlan sold a 7mm suit that was also great for a similar price. If you go to a baggier fit in a neoprene suit, you'll have room for very cold water undergarments, but you now have a baggy suit with more drag that requires a lot of weight. It will be extremely warm and would be my choice for diving the north pole, but not so much for general diving. Other neoprene downsides are that they dry slowly, are fairly heavy, especially when wet, and pinhole leaks can be hard to find and take longer to repair.
Trilam
A trilam will be very draggy unless it fits you very well or is custom cut. And, in that case, it also becomes a one trick pony in terms of underwear. You can get a suit that fits well with heavy underwear, but then it will be overly baggy with excessive wrinkling and increased drag. Or you can get a suit that fits well with lightweight underwear, but then it will restrict your movements with heavy underwear. The downsides of trilams are the condensation issue in cold water, limited durability, potentially high cost (especially from DUI, and DUI is mostly riding on name and past reputation now, they are nothing special in terms of quality) and the lack of stretch in the material (and the resulting fit/drag issues). They are however fast drying, lightweight, and both easy and fast to repair.
Crushed or compressed neoprene
The two are not the same, but they are the middle ground between neoprene and trilam suits. They offer some inherent insulation, avoid the condensation problem and tend to be very durable. But they are heavier, and while they have some limited stretch, they have to have the baggy fit of a trilam. They are slow drying ad slower to repair.
Bilam suits.
Where trilam uses a nylon/butyl rubber/nylon "tri-laminate" material, most bilam suits use a nylon fabric with a urethane coating on the inside. They have all the basic traits of of a trilam with the exception that they are less expensive, less durable, and much less longer lasting as the bilam fabric tends to separate. The exception here is the Whites Fusion, which uses a bilaminate bladder, but one that does not appear to be urethane and is very durable.
Whites Fusion
As noted, it has a bilam inner bladder, but it's covered with an outer shell available in a few different styles from an all lycra sport skin to a lycra and neoprene tech skin to an all neoprene bullet skin. The combination of an inner "bag", which is fairly roomy, and a stretchy outer layer that provides both protection to the bladder and serves as a compression garment, allows a stock suit size to provide a near custom fit. If you get a size that allows heavy undergarments, it will also be very streamlined when worn with just a wicking layer, and in both cases it is almost as streamlined as a wet suit and swims closer to a neoprene drysuit or even a wet suit than a trilam drysuit as bubble management is easier than with a trilam. They are also very durable and easy to repair. The outer cover dries slowly if you have something other than the sport skin, but they are also comparatively lightweight and pack in a small volume. They are in my opinion, the ideal drysuit.
zipper styles
Three major options:
1) across the back. Inexpensive, streamlined (as it does not need a telescoping torso), durable, but somewhat restrictive and hard to get in and out of on your own (you ca do it, but it requires skill and technique).
2) diagonal front zip. Self entry and exit, more expensive, usually requires a telescoping torso and adds a bit more drag, and requires a cover to prevent waist belts from eating it.
3) across the chest front zip. Mostly found on Whites and Andy's drysuits. Self entry without the need for a telescoping torso - sort of a pop top approach to dry suit entry. Also requires an over zipper to prevent damage from straps.
Valves:
They only come in Si Tech and Apeks flavors. I prefer Si Tech but both are fine. Inflators are on the chest and exhaust valves can be mounted on the upper arm or on the cuff. There are pros and cons based on how you'll use the suit, but most people will be better served with a valve on the upper arm.
