Whats the difference?
Fibreglass is a mesh that contains woven glass fibres. These fibres work with the resin, they provide a bridge between the natural nail and the EC Adhesive. Fibreglasss reinforces and crosslinks the EC adhesive and gives it flexibility. Due to it's glass contents the mesh, when not fully saturated will shine through under direct light. Using a thicker viscosity adhesive will reduce the light reflecting from the mesh and will reduce the mesh visibilitie.
Silk material is made from 100% spun silk, it has a much finer weave and holds it's own weight in liquid by about 30x. It is less flexible because of the amount of EC adhesive absorbance, making it much more ridgid. It makes an excellent base for sculpting a nail, extending the natural nail to bring it in line with the others. It gives the nail a softer appearance and does not reflect the light, making it totaly invisible with correct saturation technique.
HTH