Essentially they are the same instruction, just different ways of saying it. IIRC, "line up [and wait]" is ICAO phraseology whereas "position and hold" is not, but used in the U.S.
Hope this helps.
Jason is absolutely correct here. In fact, "position and hold" was just recently shortened to that, from "Taxi into position and hold", which was directly the equivalent to "Line up and wait." You'll still see some references to it in the US' FAA Order 7110.65. That's available at the FAA's website, so have a look at that, and you'll find a plethora of information about how the US does it.