Belfast -> Scotland Ferry and Car Rental

My wife and I are planning a November trip to Ireland and the UK. We're planning to cross from Belfast to Scotland by ferry and pick up a car rental on the Scotland side for our time in Great Britain. Ideally, we'd like to pick up the car rental from very near to the ferry terminal on the Scotland side. We'll be traveling with our 2 children, luggage and 2 car seats, so we'd prefer to not have to travel far to get to the car rental place. However, I'm having a hard time nailing down the details about where the ferry terminal is and where the car rental place is. Specifically: * Several websites mention both Belfast-Stranraer and Belfast-Cairnryan ferries; however, on the Stena Line website I only see the Belfast-Cairnryan route, and it Rick's Ireland book he only mentions the Belfast-Stranraer route. * Hertz has a car rental office listed at "Cairnryan - Stena Line Terminal Building" (which sounds like it's in Cairnryan) but the address lists the city as "Stranraer (Cairnryan)".
* We also found a rental possibility on listed as "Stranraer City Office". We're quite confused about Stranraer vs. Cairnryanthey appear to be different places on a map (~ 7 miles apart) but these listings seem to be conflating them. What's our best bet for taking a ferry to a location where we can pick up a car? Thanks!