I have a 5/7mm O'Neil drysuit and I prefer it to any trilam that I have owned. It is warmer, more flexible and swims much better - essentially like a wet suit. It also requires no more weight than a7mm wetsuit and requires less weight than a trilam with heavy cold water undergarments. The snug and streamlined fit combined with great flexibility makes it an enjoyable suit to wear.
I have had my neoprene drysuit ice diving with nothing more than light polypro fleece undergarments underneath and in contrast the same undergarments are good to only about 70 degrees when used with a trilam.
A neoprene dry suit can be much snugger than a normal dry suit as the material stretches. so if you get a snug near wet suit like fit with a very lightweight fleece undergarment on underneath (I like to use lightweight polypro fleece underwear sold Cabelas, Gander Mountain, etc.) you are good to go. For the most part with a neoprene drysuit you want an undergarment only to wick the moisture away from your skin.
For over a decade I was a disciple of DUI drinking the kool-aid indicating that neoprene suits were overly bouyant on the surface and would compress and be cold at depth. That lasted until I tore a neck seal and borrowed a neoprene drysuit for a deep, cold water (upper 30's) dive. I was never that warm in a trilam even with heavy 400G thinsulate underwear.
And as indicated above, the weight requirements are no worse than a trilam. In the distant past neoprene drysuits had exhaust valves on the chest and this location resulted in substantial air trapping and led to the need for much more weight. Unfortunately the change to choudler dump valves occurred at the same time as the rise in popularity of the trilam and neoprene suits were associated with the need for lots of lead even though that need is not present if they have a shoulder dump valve.
Neoprene suits are much more comfortable and can be dove with very little air in them as not nearly as much air is neede to loft the much thinner undergarments and neoprene suits do not have the sharp/biting feeling squeeze that occurs in a trilam.
The downsides are that people (those who have never dove one) are predjudiced against them and often will not let you play in their reindeer games. Also, they are warm and in water much warmner than 60-65 degrees they are too warm. Pee valves are also a bit more problematic in the snugger fitting suit and pockets are much harder to attach as most pockets do not stretch while the suit material does. X-shorts may be a better option unless you find a company that will properly sew them on (without penetrating the material.)
A final downside is that some tech instructors do not approve of them as the material compresses at depth and the fear is that they will become too negative at depth. The argument is a very ignorant one as you have a wing for backup if it floods and you will be no worse off in a flooded neoprene dry suit at depth than in a flooded trilam and in the event of a wing failure, you still have the dry suit that has more than enough gas volume to get you to the surface, just like a trilam. Plus a small hole in a trilam means you get very wet and cold while in a neoprene wet suit it means you are now diving a 7mm semidry wetsuit and the flexible nature of the material makes it more or less self sealing so even large leaks are comparatively minor.
