Hi.
At least in Europe Polyspan (or Salzer paper) is available is 3 thicknesses. The heavies one is not good as the structure is not homogenic: it has longitudal bundles of fibers in it every few cm. Medium is best for everything. I don't recommend using the lightest, it needs much more dope to seal it properly and is also quite fragile I've used it in wing tip panels of my F1A's but won't use anymore. Weight difference between Medium and lw is minimal.
I buy mine from Mike Woodhouse's Free Flight supplies in UK.
L