Sutta/Sutra guideline for choosing actions in ethical dilemmas I am searching for a sutta which gives a general guideline which helps in making ethical decisions. There are many suttas which give a skillful action for a specific scenario but I am looking for a guideline which shows us how to choose one action instead of another.

Often in ethical decisions it is possible to see how any decision hurts others indirectly. For example: Should I read printed news or online news? Printed news involves cutting down trees and destroying habitats, online news requires burning fossil fuels and polluting which harms people. We can endlessly intellectualise about the results of actions but this is paralysing rather than empowering. I would hope that a sutta doesn't suggest to analyse all possible results of an action.

I'm not sure what form the guideline will be in but I suspect that it will involve looking at one's own mind to see the motivations for an action or to see if any green/anger/delusion arises when taking the action.

The differences between an ethical action, skillful action, and meritous action are quite slim so the sutta could refer to any of these.
Don't paralyze yourself with ethical intellectualism.  You're just going to drive yourself batty.  Eventually you do have to act.

I'm going to give you two answers.  They're related to each other.  The first comes from the Majjhima Nikaya.  The Ambalatthika-rahulovada Sutta (MN 61) (one of my personal favorites!) rightly places morality as an aid to enlightenment rather than making it an end in and of itself.  It also recounts just how destructive certain actions are to the contemplative life and, rather being prescriptive about morality, it instead places the locus of agency firmly within the volition of the practitioner.  Providing guidelines rather than dogmatic law, emphasizing reflection rather than strict obedience, and handing off moral decisions to each of us is uniquely Buddhist.  It's also why this sutta gets my vote.

To answer what I take to be the larger part of your questions - namely paralysis by analysis when it comes to morality - let's look at a Zen koan from the Mumonkan - 


  Case 38 A Buffalo Passes the Window                           
  
  Goso said, "A buffalo passes by the window. His head, horns, and four legs
  all go past. But why can't the tail pass too?"   
  
  Mumon's Comment 
  
  If you make a complete about-face, open your eye, and give a turning word
  on this point, you will be able to repay the four kinds of love that
  have favored you and help the sentient beings in the three realms who
  follow you.
  
  If you are still unable to do this, return to this tail and reflect
  upon it, and then for the first time you will realize something.  
  
  Mumon's Verse 

         Passing by, it falls into a ditch;
         Coming back, all the worse, it is lost.
         This tiny little tail,
         What a strange thing it is!



Instead of thinking about buffaloes and windows, let's instead think about suitcases and clothes.  You've seen plenty of cartoons, I'm sure, where Daffy Duck is packing for a trip.  He crams everything into a suitcase except the sleeve of his shirt is hanging out the side.  All that effort in packing, everything in there all delicately placed, neat, and even the latches close snugly.  But still that damn piece of sleeve.  

So what do you do?  Do you leave it hanging out?  Well that's annoying!  But what are your alternatives?  You could cut it off, I suppose, but that would just damage your shirt.  Obviously, the thing to do is to just leave it.  I mean, how important is it really that everything fits into the suitcase perfectly?

Morality is really the same thing.  We can devise all of these perfect systems, take special care in reflecting on our actions, but the more we pack into the suitcase - e.g. the further we expand our reflection - the more likely it's going to be that something doesn't fit perfectly.  There are always going to be loose ends.  We can't worry about it.  Unless we stop fretting, pick up our suitcase, and walk out of the door, we are going to miss our train.