Some mothers like the rectangle so they can fold them to fit best on the shape of their baby's bottom. But I have found flat diapers almost impossible to find, so you might need to sew them yourself. Gauze used to be the cheapest, and lightest fabric for I find that the light gauze doesn't hold up well, and traditional birdseye is expensive and difficult to find. Currently most diapers are made of a fabric that is halfway between birdseye and gauze. It is strong and absorbant.
Flat diapers have the benefit that they are the fastest drying diaper.
This could be a consideration if you line dry in a humid climate. They are sewn into a rectangle that can be folded into many different shapes to fit babies of all ages. Prefold diapers come lined and unlined. Unlined fit better on a new born. Avoid the ones that are lined with a foam core, as they tend to self destruct faster.