Local dealer has a F350 that is optioned 99% of what I'm looking for except it has the fifth wheel prep instead of the gooseneck prep that I need for a farm trailer. What's the difference between the two prep options?

 

I think the prep package is good for both, at least that is the why mine is. It will except either a 5th wheel or a gooseneck ball.

 

^^^What Tim said. The factory Fifth Wheel / Gooseneck Package is good for either one....a fifth wheel hitch or you can use it with a gooseneck ball. The Fifth Wheel / Gooseneck Package is one package instead of two.

This is from the 2013 Order Guide:

5TH WHEEL/GOOSENECK HITCH PREP PACKAGE (53W)
Availability:
• Optional on XL, XLT, Lariat King Ranch® and Platinum
Not available with:
• 66D Box Delete
• Drop-in Bedliner (85L) – Late Availability
• Stowable Bed Extender (85B)

Includes:
• One Frame under-bed cross member
• Five pickup bed attachment points w/plugs
• One integrated 7-pin connector on driver's side pickup bed wall
5th Wheel Hitch Compatibility:
• The 5th Wheel/Gooseneck Prep Package (53W) is compatible w/the
factory orderable 5th Wheel Hitch Kits (15K and 15L) and Dealerinstalled
Ford Custom Accessories 5th Wheel Hitch kit by Reese –
part #BC3Z-19D520-A (8ft box only). The prep package is also
compatible w/Reese Signature Series 5th wheel hitch kits updated
w/a new Leg Service Kit – Part# BC3Z-A00A25-A (8ft box only)
• The 5th Wheel Hitch Kit (15K) and Dealer-installed Ford Custom
Accessories 5th Wheel Hitch kit by Reese – part # BC3Z-19D520-A
is not released to the short box (6.75 ft. box)
NOTE: The short pickup box provides less clearance between
the cab and 5th wheel trailer compared to "long box"
pickups. The receiver centerline of the hitch should be
mounted at least two inches forward from the rear-axle of
the truck chassis. When selecting a trailer and tow
vehicle, it's critical that this combination provide
clearance between the cab and tow vehicle for turns up to
and including 90 degrees. Failure to follow this
recommendation could result in the trailer contacting the
cab of the tow vehicle during tight turns.

Gooseneck Hitch Compatibility:
• The 5th Wheel/Gooseneck Prep Package (53W) is compatible only
w/the factory orderable Gooseneck Hitch Kit (15J) or Dealerinstalled
Ford Customer Accessories Gooseneck hitch by Reese –
part # BC3Z-19F503-A (8 ft box and 6.75 ft box)

 

I have a 2012 F250 with the 5th wheel prep package and it comes with both. I love it!
I use the 5th wheel part of it to tow our trailer and the hitch is an easy-in easy-out chore. Plus when the hitch is removed there's no rails so the bed is completely flat.