The choice of a BC makes more of difference to your diving comfort than any other bit of kit save for the cylinder.
Your diving comfort depends on reducing drag. We want a BC with the smallest possible drag profile we can find. Most BCs in SA are far too big and have far too much lift with the downside that they are high profile/high drag BCs.
Below we will characterise what determines a Good Low Drag BC as well as explain some of the terminology used to describe the many different BCs available.