Personally I keep my horses out 24/7 unless they need to be brought in for a specific reason. I think they're happier, they get more excerise and in the wild, horses aren't in a stall. Also horses are herd animals and they're not in a herd if they're in a stall all day or most of the day. At least have your horse out during the day and inside at night.
When you keep your horse in a stall it causes so many medical conditions like arthritis and is really bad for their digestive tract because horses are supposed to eat a little bit all the time and in a stall they eat lots of hay at once if you don't put it in a slow feed hay net. It's proven that horses that are kept in a pasture don't buck,bolt or spook as often. Horses are supposed to be with their friends because they are herd animals and in a stall they don't get the proper exposure to other horses as they should.
In my ideal world, I would have a huge pasture with several run ins connected to a stall, so my horse could have the best of everything haha. But personally, I think that the most healthy thing is to give them as much turn out as possible