if i had to pick from the two u mention i would buy a gmt, LOL
assuming you have both, pass a small lead shot, 7 or 8 birdshot which ever sounds off, take and have fun, if no response get a gmt, or get a small split shot, apss over the loop and see the distance between loop and splitshot that which is greater , go for the gold limit your disc or none at all and dig all targets go for the gold
any detector can find a brick of gold
oh and don't forget meteorites