Solid would offer more visual privacy to the space behind it on the second floor. Spindles would give a more open feel. That's two things to think about.

And a solid wall gives you more furniture options behind the wall if that is of any importance in your setting.

Puppet shows are better preformed behind a rail wall.

A curio cabinet wall with three equal sections, each with a solid glass front facing the downstairs and solid wood cabinet doors on the back to access the place where you can store and display family treasures -- lockable if the family treasures are valuable.

-- A curio cabinet there would be easily accessible so that what is displayed can be changed with the seasons or your mood and enclosed so you don't have to dust each piece.

You could even add drawers (top and/or bottom) for things that fit in them that you keep but don't want to display. (Maybe that small foreign coin collection from your travels or the state quarters or the inherited jewelry that you never wear but that you keep for the memories.

I was going with a wall and put beds againt it. When I saw it open, I liked how I could see through the window on the second floor.

What you are comparing is two versions of the required 3ft high “guard”, one with open balusters below it and one with a wall below it. If both work as a practical matter, I would go with the general principle that open is visually better than closed.

I plastered the walls. The top one with a rail wall, the other one with railings. Which one would you go for?

"I was going with a wall and put beds againt it. When I saw it open, I liked how I could see through the window on the second floor."

Yes, it's kind of cool to be able to see out the window that's on the far wall upstairs, but if there is anything at all (furniture) between the railing and the window, it's going to block the view out the window.

If you're going to have a bed or beds against that railing, there won't be any privacy for the person/people in bed if anyone is downstairs.

If I do rails, I won’t put beds In the loft. I will make it a studio. If I ever need it to sleep someone, I can get an air mattress.

We had rails to loft in our old house when kids were small and it caused problems, especially when the hamster escape his cage. He did survive the fall but it took days to find him. I would choose a solid half wall. You will still have the view over the rail and people won't see up into studio clutter.

Oh boy, hamster fall!