Polyspan is best applied so that the grain runs parallel with the longest edge. So spanwise for wings and stabs. The only source for Polyspan that I know of is Aerodrome R/C, although there must be others somewhere.

As for the actual chemical differences between Nitrate and Butyrate dope, I have no idea. I know that Nitrate is not fuelproof, and butyrate is. Butyrate can be applied over nitrate, but nitrate can not be applied over butyrate. I have read that nitrate has more dope "Solids" in it, and therefore fills fabric and paper coverings faster than butyrate. Nitrate also makes a more compatible base for other types of paints. I have not experimented with using different brand thinners with dopes. I always use Sig Nitrate for the first coats, thinned with Sig brand dope thinner. Sig thinner can be used on both Sig Butyrate and Nitrate products. For colour coats, I use Brodak brand butyrate and Brodak thinner. I like the way Brodak dope handles. It also seems to have a higher pigment concentration than the Sig colour dopes, meaning I can thin it more and still get good coverage when I spray it. If you are experimenting with different brands, I highly recommend that you always do a test piece first.