Upper belts that are mounted to the floor can compress the spine. In a crash , the shouler belts (improperly mounted) will pull your body down into the seat towardss the floor, thus posibly compressing the spine. properly mounted, they will just hold you tightly in the same position you were in.
I'll look aroud the net for a better explanation.
As to camlock vs latch, that can be a difficult decision. Camlocks are very fast to buckle and unbuckle while latches are stupid simple. Camlocks are not highly thought of in off-road racing as the locking mechanism can get filled with dirt, sand etc and not work properly. This is where the latch style shines.
Also if you plan on any sanctioned racing, check the rulebook as they quite often specify what type(s) of safety gear is required.
Here's a quik link to show the different styles
http://www.racerwholesale.com/index.php?cPath=21
I'll look for more information.