What is a semiclassical portrait, first of all?
The AOV you want is up to you, and will probably change for each picture. Just remember that the wider you get, the bigger the change in AOV when changing a fixed number of millimeters in focal length (10mm in this case). For instance, the difference between a 55mm and a 50mm is greater than the difference between a 105mm and a 100mm, because the fixed change of 5mm is a greater percentage of 55 than it is of 105. Thus, there is a relatively large (20%) difference between the 50mm and the 40mm.
To my recollection, a 55mm lens on 6x6 is similar to a 35mm lens on a 35mm camera. For these types of pix, but a bit wider, I might want 24 to 28mm lenses on small format; probably more likely a 28 (though it depends on the shot), so I might go for the 50mm first on an SQ.
The lenses for the system are pretty cheap, Can you get both?