I have essentially the same truck (2004.5) with a PacBrake installed. I live where it is mountainous and wouldn't tow without it. Holding gears means nothing, there is no compression braking with your engine when stock. You will rely strictly on your truck and trailer brakes. In my opinion, adding the brake will allow for safer towing and make the truck easier to sell on the back end. There will be a slight increase in fuel economy due to lockup in third and less TC slippage due to higher fluid pressures the PacBrake programs for. Prior to the fifth wheel, I towed a 25 ft. TT and never once felt the brake was overkill.
My truck has less miles than yours, I currently see no scenario where I would consider trading it.