Stewartia koreana, also know as Stewartia pseudocamellia var. koreana is native to Korea. Stewartia pseudocamellia is native to Japan.

I found a total of two sources that state the Korean native is more hardy than the Japan native. I don't know much of those regions but it seems odd one would be more hardy than the other now that I look at the average temps. in Korea. Perhaps that doesn't mean anything, just typing outloud on the surface.

I tried to find more info on the Stewartia Umaine but it appears that fizzled out. perhaps I could contact one of the professors.

The natural distribution and cultivated areas of Stewartia taxa are USDA cold hardiness zones 6 or warmer. One cold-tolerant clone, named Stewartia`UMaine' (UMaine Silk Camellia), has been growing well at the University of Maine Littlefield Ornamentals Trial Garden (USDA Zone 4). The plant also has brilliant red fall color and biennial flowering. Since cold hardiness field evaluation could not provide genetic information and no other taxa could grow in Zone 4, AFLP markers were employed to figure out its genetic relativeness with other 16 named Stewartia taxa...AFLP data support that Stewartia`UMaine' is a new cultivar, which originated from a gene pool of S. pseudocamellia, S. sinensis, and S. koreana.

Otherwise, if not already familar you would probably be interested in the color illustrated article Stewartia in Cultivation, which comes up when searching for it by that title.

Maybe this doesn't change the point, but there are still disagreements/varying opinions about that.