I find that i cant breathe out of my nose when running as i merely cant get the amount of oxygen that my body seems to be demanding - where as breathing throught mouth, whilst its still down from what it needs, it is at least better.
I think I breathe through both nose and mouth. But I have a slightly different (and very irritating) problem: On at least one of my runs each week, my ears start to feel blocked, especially my left ear. I'm left shaking my head like a dog (which must look weird to passers-by), but effectively my ears don't feel very pleasant, and I can't seem to shake it. And no, swallowing doesn't help either. Any ideas?
There are some advantages to breathing through the nose, ie air gets warmed and filtered before hitting the lungs. However the relatively small area of the nostril openings compared to that of your mouth seriously limits your oxygen needs when you are exercising at a more brisk pace!
Breathe through your mouth, it's more esthetically (sp!?) pleasing also!