Look into the Andersen Ultimate 5th wheel hitch. IMHO it's the best solution to the problem if you are at all concerned about the stresses a gooseneck adapter puts on a weak 5th wheel frame.
If you never get the trailer hung up anywhere, a gooseneck adapter won't be a problem. But if you sink the trailer in the mud and need to really tug on it to get it out, that's where a gooseneck adapter might be a problem. The Andersen Ultimate eliminates this by putting the stress back up at the kingpin where it was designed to be while giving you the flexibility of the gooseneck hitch without having to lug around a 250lb fifth wheel.