Your spark plugs wont have a big effect on MPG, however the these cars can be picky when it comes to the spark plug type. I use the standard motorcraft plugs, and they seem to work well, I have heard of people having problems with some other types though. If you look up near the top of the screen there is a search button, try pressing that and typing in "spark plugs".
It's easy to change plugs in our vehicles, so you can use standard spark plugs which will give you the best mileage and lowest cost. Now if you aren't comfortable with changing plugs every 10-20k miles, then you should use Motorcraft platinum plugs which should only need changing every 60-100k miles. Regardless of how often you change your plugs, you should change your spark plug wires as often as possible. I highly suggest lifetime warranty plug wires, and changing these every 2nd or 3rd oil change.