Goosenecks are generally used for agricultural-type and/or off road applications as the ball mount will give more articulation. Therefore, most horse trailers, etc. are goosenecks. For whatever reason, goosenecks are also popular among "hot-shot" rigs, pickup-based car haulers, etc. The gooseneck doesn't take up as much bed room as an installed 5th wheel hitch, although a number of modern 5th wheel hitches can be removed from the bed pretty easily when not in use.
5th wheel hitch designs are normally used for RVs and larger commercial (think over-the-road Class 8 semis) trailer applications on the road. The modern hitches provide 4-way articulation, but probably not to the extent that a gooseneck can provide. The main advantages of the 5th wheel hitch are (1. ) positive locking kingpin versus ball mount (no safety chains required) and (2. ) larger load carrying area with the loadplate versus all pin/hitch loads being concentrated on the gooseneck ball. As a matter of interest, my RV will hitch directly to a Class 8 tractor's 5th wheel hitch with no modifications - pin dimensions are the same.
Rusty