A friend of mine owns a MacBook (13-inch, Late 2009), 2 GB RAM. Because he wants to format it, for best performance, recommended to install Sierra, El Capitan, or what previous version? I think El Capitan. I'm in doubt between Sierra and El Capitan, who is better? I'm still recommending to switch to 4GB of RAM, so think as if it had 4GB. Thanks!