body type really does make a difference.. I took a football player/body builder snorkeling once.. he wasn't a good swimmer, true, but with a full 3mil suit on, a waterski life vest (granted, wasn't quite matched to his weight) he still didn't float!! I couldn't believe it. He didn't need weights - he needed more floatation. It always amazes me that so many people that respond to questions of weights don't ever consider the amount of fat one has. Myself, fresh water, no weights, 3mil suit, puts me nuetral at about -25'.
When I take someone out freediving I like to have them weighted to just float on the surface without treading, then as they progress, find nuetral at -25 or so feet with a deep breath hold. I always like being able to recover without wasting energy treading water and I like being able to float on my back for a good snorkelless breath up, even without a tube or float.