I understand your reasons for not wanting to use timber, but if you went with an exotic wood like Ipe or Cumaru, they're basically impervious to termites. They're also really strong. The direct sunlight would cause the color to go gray, but aside from that the railing wouldn't suffer any structural damage.

I can't imagine termites being much of a problem on a 2nd story, or maybe it's a regional thing? Your house is probably termite-protected anyway. Since you mentioned that it gets direct sunlight, think about how hot that metal will get. I'd go with wood and stain it.

A wooden railing is definitely the better choice. Vinyl railings wouldn't be suitable for the second floor. You could also consider composite railing; it is strong and durable too.

Are you looking to do pickets , glass or cable? We do almost all aluminum railings in Washington state, with stainless steel cable being the majority. We may not get as hot as you in California , but have never had an issue with it getting too hot. Definitely less maintenance.