If the version on your computer is up to date, I would recommend copying that installation over Steam's. Otherwise you'll get the base Steam installation and the client will update when you run it.
There's no real difference between the clients other than install location.
"Don't let them promote you. Don't let them transfer you. Don't let them do anything that takes you off the bridge of that ship, because while you're there... you can make a difference." - James T. Kirk