Unless more privacy is needed upstairs, I wouldn't put in a pony wall. As long as your are sure your children won't climb on them, I would use horizontal wood.

I had an interior designer come in for an opinion and she suggested the same thing! I am reluctant to do that, because our original goal was to make the ugly staircase "fade into the background" as much as possible. But this look does match the family room as we have natural wood trim/moldings and a two story stone fireplace. Thank you for the photo!

That's an interesting option. I wonder though if that might be too contemporary for the space? I live in a traditional colonial and the family room below is somewhat rustic with a wide paneled hardwood floors and all natural wood trims.

if you would like to use the loft as overflow sleeping space for guests, or if the man cave will tend to be errr... a hot mess, a pony wall is good. looking at the bottom of furniture isnt great, even if its tidy.

you could split the difference, do pony wall for the section to the left, and then a more open rail on the right and down the stairs.