Trilaminate

Lighter to carry.   dries out faster (after washing), easier to get on &
off, greater variety of thermal insulation levels (dependant upon
undersuit); thus can be used for all temperatures of diving, and lastly more
colourful range of materials.

see www.hammond-drysuits.co.uk

TonyH

From the GUE website:

Drysuits

Divers entering cold water or engaging in more extreme exposures will often
choose to utilize a drysuit for increased warmth. Drysuits are typically of
two primary types: shell-type or neoprene. Neoprene drysuits may come in
different thicknesses, adding to their insulation. However, a thicker
neoprene suit adds buoyancy, increasing a diver's need for extra weight. One
of the disadvantages to neoprene suits is that they will become thinner at
depth, decreasing their thermal advantage and requiring additional
insulation. This insulation, coupled with the positive nature of the
neoprene, can further add to the buoyant nature of the suit. At shallower
depths, the crushable tendency of neoprene is less significant, giving the
suit more natural insulation.

One of the greatest advantages to the neoprene suit is its limited drag.
Neoprene suits are generally very similar in drag to wetsuits, especially
when fitted properly with limited excess material. However, neoprene wrist
and neck seals are not typically as effective as the latex seals found on
many shell-type suits and generally allow some water migration. Individuals
may chose to use latex seals on a neoprene suit or utilize a double wrist
seal system: a latex seal that is covered by a protective neoprene seal.
Shell suits are constructed from a variety of materials and rely on
undergarments to provide insulation. These suits typically depend on latex
wrist seals which are generally more effective at preventing water from
passing into the suit. Thinner latex seals are more effective barriers to
water yet are more easily damaged, while thicker seals resist damage but are
less effective at preventing water migration.

The drag associated with a shell-style suit depends on the material and the
fit. For example, Viking drysuits are constructed from a thick material yet
tend to be prone to a high degree of drag. The DUI TLS-350 suit has a
telescoping torso design that allows for a tighter fit while maintaining
ease of entry. The suit is also made from a trilaminate design that allows
for a durable and flexible material allowing divers a greater range of
motion. Generally speaking, shell-type suits are more flexible and less
prone to water leakage than neoprene suits, yet can develop punctures or
failures that allow significant water leakage. Drysuit manufacturers are
always experimenting with new, stronger materials. In 2000, DUI introduced
its CLX-450 suit with a shell of laminated Cordura. The CLX appears to have
durability qualities more in line with compressed neoprene but with the
flexibility, rapid drying time, and lighter weight of the TLS. Some
undergarments incorporate insulation materials such as 3M Thinsilate that
maintain most of their insulation properties even when flooded. Some suit
manufacturers try to combine the durable advantages of neoprene drysuits
with the consistent buoyancy of shell suits.

Crushed and compressed neoprene suits can limit the buoyancy changes of
conventional neoprene by pre-crushing the material. The pre-crushed material
provides more insulation than a fabric suit, yet is less buoyant than a
conventional neoprene style suit. The DUI CF-200 suit is one of the most
popular of these crushed neoprene styles and is widely known as one of the
toughest suit materials currently used. However, a crushed neoprene suit is
very heavy and takes significant time to dry. Furthermore, even crushed
neoprene suits are less flexible than shell suits.

Additional Considerations

 1.. Drysuits that are not form-fitting will add significantly to a diver's
drag. The diver should attempt to limit the air in his drysuit to reduce air
transfer and increase proficiency.
 2.. The diver must ensure that he is weighted such that he can remain
negative with empty cylinders. This may require substantial weight for a
diver using a drysuit. This weight may be in the form of a heavier stainless
backplate or from weight placed under the diver's tanks.
 3.. Some manufacturers have chosen to use a non-standard connection on the
drysuit inflation inlet, limiting a diver's flexibility. The diver should
consider replacing non-standard connections to allow the exchange of
buoyancy compensator inflation and drysuit inflation hoses.

Having used both over the years I highly recommend a trilam suit over
the neoprene for reasons already stated (lightweight - fast drying -
easy on & off - etc.).  The only drawback to the trilam suit is if you
dive broken-up wrecks with jagged edges you must be extremely careful
that you don't cut the suit......not that you can't do the same thing
with a neoprene suit.....you can.....I've seen it happen.
Trilam or neoprene??  My vote is for trilam.

Thanks everyone (well, almost everyone!)
Very helpful.