If you are going to build this trailer, I will assume you know how and where it will be used. We sell trailers, and every large open trailer we sell is a gooseneck. There are generaly three styles two choice from. You have the ADJUSTABLE HEIGHT KING-PIN, INVERTED 5TH WHEEL, AND THE ADJUSTABLE HEIGHT BALL COUPLER (AKA- Gooseneck).
Alot of travel trailers have King Pins (5th wheels), but it seems few of them are adjustable. This is important if you are going to build a deck-over with this type hitch, they have to be adjustable, to be fully functionable. The Gooseneck hitch is by far the most popular in this type of trailer, it does offer much more flexibility. The weight rating is higher, most quality gooseneck hitches offer a rating of 25-30K lbs. Also having the bed be relativly free to use is a huge consideration, most folks need to carry additional items that they don't what on or in there trailers (gas,fuel,tools,ect). The trailers track fine on road and are great in off-road situations, the BALL allows much more moevment side-to-side then a fifth wheel plate.
If nothing else, think about re-sale value. I know you might not think you will sell it, but things change and who knows what tommarrow will bring? A fifth wheel open deck-over is almost useless to 95% of the people who would need one. So that might be a consideration for you. Also if you do decide to build this trailer yourself, remember to keep all the recipts for the matterials you purchase, they are needed when you try and get your trailer registered. Also check with you insurance, alot will not cover home-made trailers. And send off and get the required information from your local and state transportation agentcies.
Good luck, and post a picture when done.