Wetsuit Comparison

[AandA]

Sagacity,
The question you pose as to which is warmer a two piece or a one piece brings up other interesting variables that you've noticed.  In my opinion the suit that will keep YOU warmer ??? ???...... is the one that fits you better.  Yes indeed a two piece will place more neoprene around your core.  Will that keep ya warmer......maybe.  That would depend on how well the suit fits and how much water is allowed to travel between the layers. 
Theoretically the farmer john style should keep you warmer.  However the theory part just does not fit the real world....

I've stayed warm and gotten cold in both styles.  At this point I prefer fullsuits, but that’s probably just personal preference talking.

I also dive wet ;D.  I have a 7mm semidry wetsuit that is one-piece.  This wetsuit allows very very little water flow (sometimes my swimsuit will be dry when I'm done. Not often but sometimes.)  I always wear a 1mm skin under the suit.  I find this suit to be just fine for diving above the thermocline, and pretty good for brief (under a half hour below).  For the real cold water, under the ice, deep, superior, etc.  I layer a 3/2 fullsuit between the 1mm skin and the 7mm semidry.  This of course has the effect of providing me with an 11mm full suit.  For my head I'll double up on hoods.  For my feet I'll wear two pair of neoprene socks under my 7mm booties.  For my hands I'll even wear a pair of three finger mitts over my gloves.  I stay quite warm and toasty 8).