I think i will dip my foot in the water and make an effort to answer this question.
A one piece suit is a better suit for racing or trackdays or if you want a safer leather suit for street riding. When a rider goes down in a wreck the rider rolls or slides on the ground and when this happens the friction with the ground rips your clothes off. A leather suit is the insurance against most of this happening to you. A two piece leather suit is better than wearing street clothes but it is possible the suit could rip apart, so a one piece suit is the best choice to give you protection when riding your bike.
The hump on the back of a suit is for stability of your head, it will reduce the amount your head can bend backwards, it also will give a smoother flow of air over the rider. You will find this on one piece and two piece suits as well as leather jackets.
Most jackets one, and two piece suits will have protection built into the jacket and suit.
A two piece suit if good, a one piece suit is better for rider protection.
I'm sure I left some info out but I think you get the differences between the two styles of suits.