I think much depends on where you are and what your horses like.
Right now, I’m in NC and my horse lives outside 24/7 in his own pasture (neighbors on both sides), but shares a run-in with one neighbor. The run-in is very well constructed and faces East. This direction is ideal here because it blocks the north wind in the winter and the South sun in the summer. It also has lights and fans. THIS horse really prefers to be outside. Some pastures have 2 horses in it and building a very small paddock attached to the run-in really helps if you have to separate horses for feeding - one learns to stay in, one learns to go out and you can close the gate or restrict movement or grass intake for a horse. Alternatively, if you have time, you can stand with a flag keeping one horse away from the other. They catch on really quickly!!
Of note, everyone has a dedicated stall in an old Pig barn that a previous owner renovated into a horse barn. The horses are rarely in the barn, but are when there is adverse winter weather. If your horses are barefoot and furry, they’ll likely do fine in a nice run in. If they are shod and blanketed, it’s good to have a dedicated stall in cold, windy, icy winter weather.
In TX, my previous horse lived in a shed row stall but turned out in her individual paddock all day (was brought into a barn in PA and NM). She LOVED her stall and her individual snooze time in it. I tried pasture boarding her for 2 months once and she was miserable. MISERABLE without her stall and being turned out with aggressive mares (she was at the bottom of the herd dynamic).
I suggest visiting people who have barns in your area (shed row, run-ins) and asking them what they like/dislike about their design. My good friend is currently renovating a “storage barn” into a real horse barn, and she got so much information and help by visiting people. One neighbor regularly comes over to evaluate how their renovation is going, since he built his own barn but neither my friend nor her builder built a barn before. Soooooooo helpful!!
Good luck!