To me the most important thing to consider is depth of field, and I like to make portraits using an aperture around 18-25mm.
25mm aperture with a 50mm lens is f/2 (50/2=25). With a 35mm lens it's f/1.4 (35/1.4=25). Magically, 50mm at f/2.8 = 18mm, and 35mm at f/2 = 17.5mm. Using a 100mm lens I need f/5.6 or f/4. Etc.
This is completely regardless of format. If I were using my 5x7, which has a 210mm lens, I'd be using f/8 or f/11. 210/8=25mm and change. Etc...
Attached are a couple of portraits, all with different lenses, but similar aperture. To me the lens focal length doesn't matter so much.