I have used both neoprene and trilam suits extensively and there are pros and cons.
neoprene suits:
In the distant past, neoprene suits were failry baggy and had chest mounted inlet and dump valves. It took a lot of weight to sink them. However currently available neoprene suits are cut with a much snugger fit and if you get one, get it with the idea of wearing relatively light undergarments underneath. No need for really bulky weezle or thinsulate underwear - just a medium weight poly fleece will be fine. In that case the weight requirements for a neoprene suit are no greater than with a trilam with heavier underwear, but comparable warmth compared to a 7mm or 7mm/5mm neoprene suit with lighter underwear.
In my case, my weight requirements did not vary by more than a couple pounds with either a trilam or a neoprene suit when dressed for 35-40 degree water dives.
In really cold water I prefer a neoprene suit even with the lighter undergarments it feels warmer and kept me warmer. The old DUI propoganda was that neoprene suits cmpressed at depth and were colder than a membrane suit where all the insulation was in the under garment. I never found that to be the case in 35 degree water to depths of 150'.
I noted that condensation was not an issue in a neoprene suit as it can be in a trilam. I also noted that the neoprene suit was more comfortable when operated with minimal air as the "squeeze" never had the same biting quality it had in a trilam. Finally, the neoprene suit swam virtually liek a wet suit with no issues with shifting gas. You could go straight head down with no feeling of air migrating to your feet. Neoprene suits tend to be much more streamlined and have a lot less drag than a trilam in the water due to the slimmer cut. The slimmer cut also allows a full range of motion due to the stretch in the fabric. If a neoprene suit leaks, it becomes a very efficient semi-dry wet suit, while a wet trilam is just plain cold.
The downsides are that the neoprene suit is heavy, even heavier when wet, and leaks if they occur are be harder to find and repair. Zipper options are limited to across the back entry and the suit probably won't last as long - 400 or so dives. But on the other hand, you can get one new for $500-$600, 1/3rd the cost of a new trilam suit. Finally, they are not well suited to warm water as they are at their coolest still warmer than a 7mm wet suit.
Trilams:
Trilams are more or less the standard as they offer a good compromise. They are lightweight fairly durable, dry fast and are easy to repair when they leak - and they do tend to leak more due to more stress on the seams, thinner fabric more prone to getting pinched and holed, etc.
Trilams last longer but cost more, so in the end it is a wash on a cost per dive basis. Self entry modeles are available but with diagonal across the torso zippers you need a longer torso which adds to the baggy and draggy factor. The exception here is the Andy's/Whites shoulder to shoulder front zipper which allows self entry with no need for an extended torso. And, as it is just a non insulated shell suit, you can wear one in fairly warm water and not have a heat stroke.
All that said, I switched to a White's Fusion last year and in my opinion it is the best suit available. It allows the comfort and swimability of a neoprene suit with the light weight of a trilam at about 3/4 the cost of a trilam suit. It would be my choice in all but near freezing water.
