With ref the multi tine forks that are refered to as Potato forks, they are really for loading potatoes ie; shovelling them up rather than for digging them up. A potato digging fork would usually be a 4 tine fork with wide tines that are flat at the front and "diamond" at the back with a good curve to the tines to get behind and under the root of potatoes.
The digging fork in the Fiskars range has just this type of fork head and has proved to be very sturdy although the company regular changes their handle designs and I haven't used their current style.
"Dori"