First Hyperstretch, there is no such thing as a free lunch. I do not nor have owned one. There are consistent postings of rapid warmth loss, velcro attacks and overall short life. If you fit into one such that is very stetched (claims to 250%) it's obviously getting thinnner before you even get wet. The properties that allow for stretch would seem to also make the suit vulnerable to compression and loss of insulating capability at lesser depths. You will hear from divers that love them with the most common claim being comfort and easy donning. If those are the top priorities then so be it.
There are some claims of higher density neoprene, Rubitex also shows up as premium maker of material and there are variations as you mentioned of gas content. I have also heard of clay fillers being used as opposed to pure base material. These details are very hard for the consumer to get hard facts on a brand by brand basis.
So what is important? Fit, seals and thickness in my opinion.
The suit must fit you closely and some light compression is a good thing. Make sure it's comfortable in the chest and will not interfere with your breathing. Your arms should have full range of motion and you should be able to bend over. Getting into it should not be a circus act though if it's your first time it may take some practice. Start by getting your legs in up to your knees and get that just right and roll your way up. You will be wet in a wetsuit but water volume should be minimal as close to a film as possible.
You want internal rubber surface seals in the calf and forearm area as well as a smooth surfaced collar that you can adjust with velcro. These will help keep water from flushing in and out of the suit.
Lastly is thickness of material 7mm is the thickest common material and suits can be configured to provide 2 layers on your core. I prefer a fullsuit (like a jumpsuit) with a hooded step-in vest. The farmer John/Jane and jacket are the other common option.
You might look at the Bare Arctic items either the SGS of basic. You can mix and match items depending on conditions.
http://www.bare-wetsuits.com/baresh...asp?dept_id=230
Have fun
Pete