outdoor - but with a high roof and vents and most improtant a big overhang. Current stable has one - stable in yard I am moving to on Friday doesnt. Not going to like winter mucking out and all grooming will end up being done in the stable if the weather is bad so dustier for me!
Mind you even with the big overhang I still had snow on the horse each winter if the wind was strong and in certain direction! Interesting to see that with a good rug on it didnt melt!
However, if I had to choose one overall I think I would opt for the outdoor. Better ventilation as others have said and I have found horses to be happier in them, more to see and watch I think (rather than just watching the horse opposite them they can see whats going on outside which must be a bit more interesting!)
I did have my horse at a yard with a big barn with 25 stables inside it. It was lovely when the weather was bad as you could just tie up on the walk way and not have to face the elements, but it wasn't quite wide enough to walk another past whilst you had yours tied up which was a bit annoying, also the horses could get their heads over the walls seperating them and bite each other if they really wanted to, especially at feed time.
I've tried him in the outdoor and he seems relatively settled, I guess it's a matter of making a decision and running with it. Some of the set ups from posts here are superb, very jealous!
Outdoors for me. She gets to see everything that's going on in the yard,lane and the farmhouse, so always something to watch. Mind you this year with the snow we had to dig her out of a 8 foot snow drift which was not good.But all in all I prefer outside.
Ventilation depends on the design rather than whether it's a barn or outdoor stables, outdoor ones with low ish roofs can be as stuffy and cobwebby or more than a portal barn, it all depends on the design/ situation.
We have a 7 stable indoor barn and love it - the stables are all in a circle so they can see each other with walls seperating them so they can groom over the top - we can keep the main barn door open if they're in during summer and close it to keep rain/snow out during winter ... my last yard was open stables and didnt like it