You can run Lua from your command-line interface, "lua filename". You can see the command in the output I provided. Anyway, as we've determined, there is no significant performance difference and you should use the whatever provides better readability. In most cases, using ".." is enough. Sometimes, it's not even possible (like when you want to print a double with only three digits after the decimal point as in your benchmark script), and that would be a fair enough reason to use string.format.

Just use whatever makes your script easier to read and maintain.