I used the hole saw, and it did a great job. Found one in stock at Sears Hardware, in case you have to chase one down. B&W's measurements were spot on the money.
Be sure to get all the metal chips out of the bed immediately after cutting the hole. Otherwise, they will rust within a day or two and stain the paint on the bed floor (glad I had mine Line-Xed after installing the hitch).
Rusty