We use the eMMC because it is faster. But if you want to evaluate different operating systems, you're better off (in my opinion) turning the eMMC off and just working with a pile of SD cards with different images.
Be careful writing to the SPI ROM. It is very hard to recover from trouble there.
To directly answer your question. NetBSD. It takes more patience to use it, but it's worth it to me. NetBSD pretty much does what I want it to do, not what someone else decided that I should want it to do.
I have two PBPs, one from early 2021 and one from the mid-2022 production run. Both of mine run NetBSD. I have (or had) a micro-SD card with "Debian Desktop", the pre-Manjaru default PBP OS, for when I need to do things that NetBSD won't easily do. But I can't remember the last time I needed it. I prefer that over Manjaru. My wife has an early PBP from the second production run, and it runs the latest Manjaru. It doesn't get updated. I update it maybe twice a year, and usually an update will brick it, and I get to sort that out. I expect that if it were updated regularly this wouldn't happen.
Be careful writing to the SPI ROM. It is very hard to recover from trouble there.
To directly answer your question. NetBSD. It takes more patience to use it, but it's worth it to me. NetBSD pretty much does what I want it to do, not what someone else decided that I should want it to do.
I have two PBPs, one from early 2021 and one from the mid-2022 production run. Both of mine run NetBSD. I have (or had) a micro-SD card with "Debian Desktop", the pre-Manjaru default PBP OS, for when I need to do things that NetBSD won't easily do. But I can't remember the last time I needed it. I prefer that over Manjaru. My wife has an early PBP from the second production run, and it runs the latest Manjaru. It doesn't get updated. I update it maybe twice a year, and usually an update will brick it, and I get to sort that out. I expect that if it were updated regularly this wouldn't happen.
:wq