What you want in a dry suit depends on the diving you are doing, and your particular imperatives.
Laminate suits are more useful in variable water temperatures, because they don't insulate, and you can use whatever thickness of undergarment you want with them. They also have no buoyancy change related to the material itself. Bilaminates are more fragile, and trilams more expensive.
Neoprene suits have more intrinsic insulation, but may not be comfortable in warmer water. They have buoyancy changes similar to wetsuits. Depending on the type of neoprene, they may or may not have some stretch that permits greater mobility. They are more streamlined because they fit like a wetsuit. Their biggest problem is that they lose insulating capacity with depth, just as wetsuits do. They TEND to be cheaper than laminate or compressed neo suits.
Compressed or crushed neoprene suits have some intrinsic insulating capacity. They tend to be very tough. They are heavier than laminate suits, and dry more slowly -- this can be an issue if you travel to dive. They do NOT stretch, and therefore must be cut generously enough to permit motion, which means they aren't as streamlined as a full neo suit. They tend to run at the expensive end of the price range.
The last type of suit is the Fusion, which I believe is marketed by ScubaPro in Europe. It's a hybrid, with a laminate bag as an inner layer and a compressive outer layer. Because of the compressive outer layer, the inner bag can be oversized, resulting in good mobility. Because the outer layer is stretchy, it can snag and let go, and protects the inner bag, so the suit is hard to hurt. You either love Fusions or loathe them. I own two. They sit at a very moderate price point (at least in the US) and are quite durable.
As far as back versus front zip, the biggest issue is convenience. Back zip suits are difficult to do without a buddy; front zips can be truly "self-donning". Another consideration is the design of a back zip, and where it places the dump valve. You do not want a dump valve on the front of your suit!
