Whichever you're more comfortable with and provides the features you want.
Define "sandboxed"
Application can only access a limited part of the system? = use flatpak or build a container/VM image using the nix pkgs.
Application can be uninstalled completely and has separate libraries? I prefer nix.
Nix apps are not sandboxed and you have no control of what resources they have access to or don't, unless you wrap them with some other program
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I'd say Nix requires some experience, so if you are new to Linux, definitely go with Flatpak. I believe Flatpak also provides stronger sandboxing.
Flatpaks are easier to use in most distros. If you're using NixOS, then Nix of course. But if you want to do a lot of CLI stuff, then Nix may be better too.
