Linux, but if there is ever an option to have one fewer services and/or root privileged things, I will take it.

Ok cool! The reason I asked is that the multi-user setup has more benefits on Linux than on macOS. That's because on Linux, the full build sandbox is actually available, and the store's immutability is enforced by default.

The tradeoffs are obviously yours to consider. But the normal Nix build sandboxing helps protect you from nasty things like crypto miners in setup.py or whatever, as well as improving reproducibility.

That's less relevant on macOS where the sandboxing story is not so great.

Personally, using Nix with a daemon seems like a better setup to me but adding another highly privileged process unnecessarily is obviously a real security concern. There is some ongoing work, btw, to reduce the level of privileges that the Nix daemon needs.