I have used a couple in my life. The one I currently own was purchased from a plumber who retired and no longer needed it, so I don't know what brand it is, but in my opinion there is one key thing to look for when buying one. Make sure the tip has a slightly larger diameter than the shaft. If the shaft is just a rod with a point, when you find moisture or evidence of a leak under the ground, pulling the rod out to check it will wipe almost all the water off of the rod and it will be very hard to notice that you found the leak area. If the tip is slightly larger in diameter, when the tip section gets wet and you pull the rod out of the ground there will be around an inch or two of the rod just above the tip that remains obviously wet.
I don't know much about different tip styles and what works best for each type of ground, but mine has a hardened steel tip that looks like an upside down teardrop and I would be a bit surprised if a different shape worked any better.
