The kind of cap (wig liner) you should wear depends on how much biohair you have, since the point of the caps/liners isn't to keep the wig in place, but instead to help restrain the hair underneath the wig.
Typical rules of thumb on this...
Fishnet caps = if your hair is long at all
Nylon caps = very short cropped biohair
Most folks with appreciable amounts of biohair won't want to use nylon caps. They'll just slide around.
Fishnet caps don't do much to protect your scalp if your hair is buzzed or cut very short. However, these tend to have a nice band of elastic at the perimeter to keep them in place, which is something the nylon caps lack.