Wow, thanks so much for your reply @melissaharlowvo! I really appreciate it!
Since I made my first post about this a couple weeks ago, I've done some more research into what kind of scripts I should use for my demos. I found a nice detailed article that discussed demos for different voice over genres. I learned from it that public domain literature is more often suited for VOs looking to record audiobooks. At that point, I actually began to wonder if I wanted to make audiobooks the primary genre my demo would focus on. As I mentioned before, I was unsure about it at first, but I had recorded several bits of text from public domain fiction by then, and found I really enjoyed it. I felt intrigued at the idea of doing audiobooks for a living.
If I'm just starting out as a working VO though, your advice would be to start with the commercial demo and build up an audiobook one as I gain more experience?
I admit it never occurred to me that VOs could create all the script for their demos. As I've discovered through research however, there are plenty of resources out there for getting tips on writing scrips in different genres. For my narration demo I did record text from five pieces of creative writing (mostly fiction) that I did in the recent past. It seems it would be worth my while though to get into the habit of writing my own demo material in any genre. It would likely also give me insight into the creative process of my clients.