I agree with Sal_E. Everybody says shavings which is one of the reasons why people use it for a horse that's chesty - but if you've got some good, well cared for straw, that can be good too. I would've said shavings though.
The shavings our YO sells (from the local sawmill) are very dusty so mine is on bedmax - a wee bit expensive but dust free. She would eat straw so thats not an option!
We use "dust extracted" shavings at work which always, without fail, make me sneeze when opening a new bale (and sometimes when shaking old bedding). I know it's not the straw as I do the shavings boxes before the straw ones! Our straw is usually very good and much less dusty. We have paper at home which beats both of them though!
I would say shavings, on the basis that tara has straw and harry has shavings and when i do taras bed it makes me sneeze and when i do harrys bed it doesn't
Good straw is better than all but the most expensive shavings.
My horse doesnt cough on straw. Out of everything else ive tried (different brands of shavings/hemp) only Bedmax and paper are dust free enough for him.