The fit of the dry suit is far more important than the brand name, so long as the suit in question is well made. The brand name may play a role in the available options, for example you can only get ZipSeals on a DUI brand suit. Other suits have their specific attributes, but generally any well made suit from a major manufacturer will be fine.
If you get a trilaminate type of suit, sometimes called a "bag" suit, then the undergarment will play a huge roll in determining warmth. If you get a neoprene, crushed neoprene, or compressed neoprene suit then the undergarment will still play a role in your warmth, but the suit itself will also have a part to play in your overall warmth.
For me, I happen to be the poster boy for the DUI extra large size. A nearly perfect fit in a stock suit. So guess what kind of suit I have? Your size and shape body will determine what suit will fit you best. Of course most of the major brands offer custom suits, and some do not charge too much more for custom. Do some checking with your local shops to get a little more insight. You can also check with local divers you might see on a dive boat and get their opinions as well.
Good luck,
Mark Vlahos