All things being equal, the bare metal (stand-alone/no OS) implementation will be more efficient as the CPU is at your sole disposal.
As operating system gives you additional capabilities (multiple processes, interprocess communication, filesystems, etc.) but it comes with additional overhead of the kernel, device drivers, task switching, etc.
However, you may reach a limit with what you can do with a single thread elegantly.
It also depends on how you define "better" (e.g. CPU utilization, time to market, development effort, maintainability, scalability, feature set, etc.)
