Picture comparison with some observations
Just pictures: Nialma v. Propercloth
You can draw your own conclusions. They both offer Thomas Mason fabric. I think PC uses more measurements. The forearm is a little thinner on my Proper Cloth shirts.
The buttons are pretty similar.
Nialma is cheaper with their deals.
Propercloth uses some double needle stitching on the side seams. Nialma does not.