the thinnest would be best for what you want to do.
The thinnest we have is 75 micron (0.003"). I wouldn't use anything thinner. Thicker can be used but it depends on the light source. If a single bulb or the sun is used, then the thickness isn't going to make a lot of difference but if a large light source such as several UV tubes is used, the light can come in at all angles so the thinner, the better to reduce softening of the edges of the image,