Lace vs silk is all about personal preference. Some people don't like lace because the material is to thin some people don't like silk because it is too thick. This will definitely have to be a decision your stylists or whomever is installing your weave will make. No matter which one you choose the most important factor is that it is put in to look natural. So she (your stylist) must tell you which one she is capable of working with so that she can put it in to look as realistic as possible.
Silk Closures are made of a thicker fabric than lace. Some people prefer silk because the fabric will look as if the hair is growing from your scalp and the base is thick enough to hide the braids underneath. Silk Closures normally run light in color so you may have to tint the fabric to match your skin color or use foundation that is the color of your skin in the parting area. Some silk closures give a thicker appearance than lace around the hair line leaving a line of demarcation.
Lace Closures are thinner and give a more flatter and seamless attachment especially when glued around the perimeter of the hair line to blend in with your skin. It can also be sewn down without glue by a skilled professional. Some people believe that you can see the braids underneath the lace when looking directly at the unit so wearing a flesh colored stocking cap is sometimes recommended.
Another concern when purchasing a lace closure is the knots being too dark. If the knots are too dark at the root of the base then the unit will not look as realistic. Bleaching the knots is recommended to give a more realistic look, the objective is for the hair to appear as if it is growing from your scalp.