I'd be happy to get you tuned up or give you some ideas. If you would like to send some pics to http://tuneland.techno-zone.net/ I can take a look at what is going on for you. I'll probably have a few questions so you might want to start a thread there in home audio.

good luck

good listening

Interested to hear Michael's thoughts on this since I was struggling with a similar issue.

Here is my take, everything you read tells you that setting you system up on the long wall is generally a bad idea (from what i read, you are more likely to have more drastic peak and nulls in your frequency response than if you set up on the short wall. Also if you have an open wall or different materials for your side wall it can create an imaging problem (that’s actually my issue now as I have curved glass side and even with the curtains closed there is still a build up of HF sound there, but that’s a separate topic – also have free standing early reflection panels which helps but doesn’t completely solve it). 

With that said, your short wall has two windows on it which can create their own set of issues. I would worry about how your speakers are going to interact with those. I think if you had shutters or curtains on it, it would go a long way to improve that wall as a starting point.

Basically in short:

1. you are more likely to run into boundary interaction issues with the windows, but I think there are a few things you can due to mitigate that (curtains or blinds to start) and you could always experiment with absorption and diffusors behind the speakers).

2. if you chose the long wall you are more likely to have imaging issues and potentially a more uneven frequency response.

I would recommend starting with 1 and see how the issues with the speakers and the windows are, since you are at least starting with the preferable setup and can try to tune from there. As a note, I have always been forced to setup on the long wall so if you want to go that way, it is doable and you can fix a lot of the issues but I have always struggled with imaging and my room is pretty well treated. Also, somewhat unrelated but I tend to think this way, if you set up the long ways, you always have the option of having a drop down screen and projector if you want to use the room for audio and video. (the extra length with help when you go looking at projectors as most of them require a good 10 to 15 feet to set up properly). 

The best looking position which makes sense furniture wise and makes the best sound music wise is the answer.  In a home environment unless you have the budget required to tear out walls etc you have to work with what you have.  Test a lot before committing to a full build-out.

It is a good idea to do a few trial setups using different acceptable positions and let the room tell you what it wants.  I usually bring a lawn chair with me and sit and evaluate for a week at least...

A few select room dampening tweaks to get rid of a few really bad reflections will yield a surprising improvement as long as you use your ears and avoid killing the liveliness and middle channel effect "focus."  The homeowner's tolerance for Sonex or at least some fiberfilled fake "paintings" will be a big plus if you can get them to sign off on it. 

A lot of this stuff comes with experience.  At some point you really HAVE to have heard a "perfect" system so that you have a clue what you are trying to accomplish in your own environment...

Not one to chicken out and refuse to offer a solution and so:

My instinct would be to place speaker pair at 1/3 intervals on the plain wall which measures 13' 7".  The "Chair" would be then on the plain wall 10' 7" opposite and it would be placed obviously at the ROOM midpoint which should be directly in the center of speaker imaging plane.  An equalateral triangle arrangement (in the classic monitoring position) for speakers/chair would be in order obviously. 

And then days of  much moving of speaker and chair away from and then back to---the walls behind speaker and chair constantly looking for the spot where it all "lock in" and becomes 3D and the tone is correct in timbre.  Perhaps a bit of toe in adjustment and lateral adjustment.  In short---the Sumiko Method (Google it).

If this arrangement simply fights you tooth and nail then take another look at the room and move on to trying another position.

But this is where I would start.  And hope it would prove ideal.

Just wanted to quickly add that your best bet is trying both and seeing what sounds best. Theres a lot that will go into how the system will sound so i would play with it on both walls for a couple of days / weeks and see which one you prefer.

Also, I just want to add that I don't think you need to have heard the "perfect system" to figure out which works better. That being said it certinaly helps to have a base line but the truth is you should go with what sounds better to your ears. One recomedation I have found is when I think I found a good place I try to listen to it in a few sessions as its funny how your preceptions can shift. Also as a side note, the seating position can play just as big a part as the speaker placement so I woud experiement with both. 

I'm assuming you will quickly be able to hear which wall you prefer and then from there you can zone in on the best speaker placemetns for that wall and look at room tunning to even further optimize it.