Howdy Pard!
I'd chime in to call it both. While as a sport, you might not get 60,000 screaming fans into a stadium to watch a metal detecting competition, there are always spectators who show up to see this type of event. I've even seen newspaper folks come out to take photos and get the story for their local edition. In some towns, a metal detecting competition will be the biggest event of their calendar year, aside from the high school prom or football championship.
To the casual observer, it might look about as thrilling as watching Thai Chi, but for those who take part, it has elements of a sport. Speed, precision, luck, exertion, physical endurance (I've been to a few that were physically brutal), and of course, training all enter into play. Depending on the terrain, you can experience the thrill of victory or the agony of the feet! In many ways, it is similar to golf. In golf you dream of the hole in one. In metal detecting, you hope for one in the hole.
Some detecting competitions are combat ugly, and competition can be fierce. Conditions can be tough. Some competition hunts have the added element of rattle snakes on the field as another element of caution. Cactus is also terrific fun as hazards go, and can leave you with memories to last many days after the competition is over and done with. In Sapulpa Oklahoma last year, mud was everywhere and proved a daunting hazard to some folks.
Point being, there are hunting and fishing shows on television and you can buy DVD's at the Bass Pro Shop about those sports. Metal detecting is simply hunting for targets while fishing the dirt.
That's my opinion, for the 2 cents found in the same hole.
Blessings,
M-Taliesin