I had to make the same decision, and ultimately picked the path that put more people comfortably in the room and made the space feel more open.
You're probably looking at 4 comfortable seats regardless of whether you put the screen on the short or long wall. Putting the seats along the east (according to sketch) wall will allow plenty of space for a large screen, and also give you the option of additional floor-level chairs or bean bags, so you can get more people in.
However, if you put the chairs in the center with projection on north wall, it will be more ideal for sound.
I think that BenQ 3050 needs a minimum of 10ft for a 120 diagonal screen, which you don't have for the west wall; you want that screen size at a minimum, so you should probably look at the 2150, or another shorter throw option if you go that direction.