Few differences. If anything, I find the Plaza to have a bit more space in the toe box , but not by much. At least that is my experience. For the M-75, get your Plaza size.
I know these are on the M-75 by way of direct email communications with Alden. FWIW, I have a contact person at Alden HQ that is wonderful in responding to my requests for information and confirmation.
I also have the BB version in black shell.