Based on my own experiments with my M60s, I'm thinking your short wall will be the better place to put the M60s, because of the benefits of sitting far enough back to get a good soundstange.
My set-up is in a 14'x18' room (open at the back to adjoining kitchen), with the speakers 8' feet apart exactly along the 14' side, ie, across the room from the opening. I would like to place them 10' apart, but cannot easily get them more than 8' apart because there's a door in one corner of the 14' wall. The speakers are 1'+ from the back wall (very important IMHO) and toed in slightly. The room has hellish sliding glass doors on EACH side wall, but drape curtains along each 6' door have tamed them perfectly, and the quasi-translucent yet thick drapes block out the neighbors as well.
Key item, for me: the preferred seating is at least 10' back from the speakers. We have a little couch on wheels that we roll back and forth from about 12' back to 9' or 10', for movies and music.
This set-up works great for us. I think it's important to be back from the speakers. It seems like this would be hard to acheive if you placed yours along the longer wall at a greater distance, because in your room it will be hard to sit far enough back from them, given that you'll want the speakers 1'+ out from the wall, AND realistically one is not always seated along the opposite wall ... you move around etc.
I don't have the chance to experiment with corners, because of the door along the speaker wall ... but you may as well try that out and see. Suspect it might be too boomy, but really have no idea.
But tinker and let us know.
Birdman
