I have the Ford underbed fifth wheel prep package. The Dodge system is similar and the cost is about the same. Installing the underbed hitch system is much more expensive after the truck is delivered as the installer has to work around the bed (or loosen/remove the bed) to access the hitch install points. I'd urge you, if you're ordering the truck and have any inclination to upgrade to a fifth wheel trailer in the future, get the option. Trust me, the bed rails in a conventional fifth wheel hitch system are a "thorn" to anyone who tows a fifth wheel. They either get in the way, prevent cleaning the bed or screw up the bottom sheet of anything being carried. If you can prevent having bed rails, for a "measley $400, DO IT !!!!!!!!!!!!!!
That said, if you're never going to upgrade to a fifth wheel and you know that decision will never change, then it's $400 wasted.