24/7 turnout is the best option for healthier, happier horses IMO. Out of the 7 horses I take care of, only 4 come into their stalls in the morning to eat their grain.. as soon as they are finished I let them back out. When the weather is really bad I will pull the two that I own into their stalls for the night, but even then I feel bad about it because it's just not natural for them to be pent up in a box for hours on end! As long as they have access to a run in shelter (all 7 don't fit under ours, thus mine come in) I would definitely leave them out 24/7.
If you don't have to stall your horse... don't! Aside from it being healthier for the horse to be out, it's less work for you in regards to keeping the stall clean and stocked. My boy can mess up a stall pretty dang quickly and I'd much rather pay for feed than bedding! Less chores inside the stall means more time for you to spend with your horse.