If you are asking which material to choose, I presume you are thinking of having some bag bellows made. I think synthetic material wins hands down on the grounds of flexibility because of the sharp bending to which bag bellows are subject - a pancake shape is best, since this means (on a monorail camera) you can bring the two standards to the point where they are virtually touching without material getting in the way. If you are having bag bellows made, it is worth considering having a size which will allow about 8" (200 mm) extension (for 4x5"), since often standard bellows will allow infinity focus with a standard lens but will be too compressed to allow much if any movements.
Regards,
David