Honestly, the #1 most correct answer to this is "the suit that fits you the best". Fit is 100% of the time the most important factor in your warm, not brand or style.

7mm of neoprene, is 7mm of neoprene and it can only hold in so much warmth.

That being said, the new Aqua Lock suits from Henderson have a new method of sealing the seams, allowing for less water loss though the seams, allowing for greater and easier warmth.