Flint 350,,, First off, no seam is necessary. Neither fabric is axially directional, so they can be run either way. Published sizes and prices reflect sizes which are common, and in no way constrain what is available from Stewart. We build exactly what is requested.
Studiotek has a bit better off axis viewing cone. Ultramatte 150 has as good of a viewing cone as any fabric available at that gain. Since the Studiotek is a little lower in on-axis gain, the cone is wider, which also translates to slight improvement in white field uniformity. The Ultramatte 150 has a slightly smoother surface texture which some viewers prefer.
So if your calculations indicate that foot lamberts are in short supply, the Ultramatte 150 is a good solution. In order to preserve maximum white field uniformity and center to edge brightness uniformity, we encourage use of longer focal lengths, so that the angles of incidence are in a tighter distribution.
Millerwill, Firehawk fabric is not axially directional and will accept or reject light uniformly side to side, or top to bottom.