But Debian testing & unstable are rolling releases...That's something Valve designer, Lawrence Yang, told us during our hands-on time with the Deck when we asked about the switch from Debian to Arch.
"So, Arch Linux, one of the main reasons, there's a couple, but the main reason is the rolling updates of Arch allows us to have more rapid development for SteamOS 3.0," says Yang. "We were making a bunch of updates and changes to specifically make sure that things work well for Steam deck, and Arch just ended up being a better choice for them."
I prefer Debian than Arch Linux because:
Debian is older,
has more packages (Arch Linux hasn't Alien Arena),
you have to compile an AUR package,
Arch Linux hasn't a (official) graphical package manager, Debian Synaptic (Syn"apt"ic),
Arch Linux hasn't a graphical installer nor a live.