We're partial to south, and as others says it is a matter of personal taste. No where on the island is "crowded" in typical American terms except maybe Front Street with golf carts sometimes. (Yeah I know, traffic is far worse all over town than it used to be but still...)
We've never stayed north but in the many vists I'll say it is VERY (too much so for me) peaceful, remote, and some would say isolated. Of course the unending pace of "progress" has slowly moved the line of isolation further north than it used to be so that is a constantly changing descriptor as well. With that said, it is beautiful, peaceful, and the reef is relatively close.
I prefer south. We normally stay 2.5 miles south and enjoy the very peaceful surroundings, with a bit more development and far easier access to town, shopping, great restaurants, sailing, etc. Amenities such as SCUBA are equally availble N or S, restaurants and shopping more plentiful (accessible) S, choices for accomodations debatably better S. And the cobblestone road goes south about 1.5 to 2 miles and that is appreciated when you are trying not to spill your beer. Either way, you can't go wrong plus you'll be returning on a 2nd trip and can try the other end if you are geo-curious.