What's the diffrence between FileUtils and system?

I care much about the reliability

Can’t see what you’re meaning. #system() is a method call, from Kernel
module. FileUtils is a standard library, which is used to work with
files. It has more features than the core class File, but I don’t know
which one is better in terms of speed. See if FileUtils is writed in
Ruby or C++, perhaps that can answer your question. Other way is make a
Benchmark. You have a standar library called Benchmark, which you can
use to test speed of running code.