DRYSUITS

Your planned diving environment will decide which way you jump here. If you dive in what we fondly call "Northern climes" you should be planning a dry future.
Your sorties abroad, to warmer seas, will probably need something different - a wet suit of an appropriate thickness. Only in the warmest waters can you consider a non-insulating suit or "dive skin".
At the top of the scale, in both thermal efficiency and price, drysuits made from trilaminate materials or squashed neoprene are hard-wearing and highly efficient when combined with good-quality undersuits. Good snag-resistance is important if you dive on rough terrain such as wrecks.
New qualities of nylon outer coverings are appearing on both trilaminate and neoprene suits to give significant extra resistance without affecting flexibility or stretch.
The main items contributing to the dryness of the suit are the seams, the wrist and neck seals and the zip.
The ideal drysuit seam has no thread that passes from the outside of the suit through to the inside, as this would provide a path for water to follow. If any exterior stitching is hidden by tape this is a good solution, particularly if the tape is vulcanised to the suit.
Most stitched seams on neoprene suits use a form of stitch that just penetrates the outer layer to bring the two sides of an already glued seam together. A similar kind of stitch may also be used on the inside and/or it may be taped.
Seals at the wrist and neck may be made from thin neoprene or latex. Latex seals are generally drier, but do not offer any insulation and can be fragile. It can be more difficult to get a good fit with a neoprene seal and the surface has a tendency to develop wrinkles with time, but it is tough, easy to repair, and provides insulation.
Some top-of-the-range suits fit a latex seal with a neoprene protective collar, a perfect solution.
Most zips are made by the same manufacturer, but it is the way in which they are attached to a suit that can cause problems. A longer zip, which usually makes the suit easier to put on, also pushes up the price. Generally the "across the shoulder" zip is the shortest and cheapest, followed by the front "shoulder to hip" and then the "around-the-neck" versions.
The choice of hood offered with the suit, attached or loose, should be based on your type of diving. If you spend long periods in your suit but out of the water a separate hood is preferable so that you can don it just before entering the water. If you can enter the water soon after dressing in your suit, an attached hood lets less water in.
Some of the newer trilaminate materials have a reasonable amount of stretch and so fit better around the limbs, as less spare material is needed to allow for bending at knees and elbows. This material might command a higher price, but should justify itself in long-term comfort.
Neoprene mentioned can be pre-compressed, as used by Northern Diver and DMS, to provide strength without the buoyancy of normal neoprene, or crushed, as used by DUI, to allow the suit to be assembled with full-thickness neoprene, which makes seaming easier. The complete suit is then crushed in a pressure chamber to give a tough, non-buoyant result.
If you find that standard sizes leave you with a droopy crotch or boots that need three pairs of socks, remember that most suits can be ordered made-to-measure for a small extra premium. Get measured by an expert; you still need to cut a bit of slack into a dry suit, but it varies with the material used.
Consider the position of the inflation and exhaust valves and make sure that they will work with your gear set-up, particularly your BC. It is tricky to move a valve to a new position, as it means plugging the old hole.