Permeability is usually measured on flat specimen and it generally has these units: [amount of stuff]-[distance through which it diffused]/[surface area diffused through]-[time to diffuse]-[driving pressure].
Assuming you use a flat specimen for your material characterization, the surface area used is obvious and is the same on both sides of the specimen. With a tube, however, the surface area is different inside and out. Presumably the actual surface you should use is some virtual surface between the inner and outer surfaces because the stuff inside has room to spread out as it diffuses through, but I'm honestly not sure where it should be. I could also see the argument that you use the inner surface as it's the smallest and therefore rate limiting.
Stated another way, in the diagram I've drawn, what is the r between r_inner and r_outer such that diffusion of blue into the environment is equal in both configurations?