They are both pretty close in sensitivity to small gold, but the GB2 just edges the GMT out on the really small stuff, having the higher frequency of the two. I watched some local guys in Adelaide using both detectors on minute pieces of gold in pipe clay around mine shafts openings, they both preferred the more sensitive GB2, but still found the GMT quite capable. Some of the pieces of gold I saw them pulling out of the clays with the GB2 were ridiculously small, almost pinhead sized stuff. The tiny 6x4" DD shooter coil was the preferred coil to get the best out of the GMT on very small gold.
The GMT was also the pick for deeper/larger targets, most likely down to the lower 48kHz frequency vs 71kHz on the GB2, but neither of them are real depth monsters. They also don't make for good coin shooting detectors, as they simply pick up too many small non-ferrous targets due to the high frequencies, literally a nightmare in foil littered areas. Both have been used for detecting micro jewellery in tot lots and dry sand, but they weren't really built with coin/jewellery detecting in mind, better off getting a seperate detector with a lower frequency for that purpose.
You also have to take into account whether you prefer auto ground tracking/balancing on the GMT vs manual ground balancing on the GB2. The GMT would make for an easier detector to start off with, plus you have the option of adjusting the ground balance manually as well, whereas some still prefer to have the full manual adjustment on the GB.
If you want a bit more flexibility on the frequency side of things, don't forget the Minelab Eureka on which you can run 6.4, 20, or 60kHz.
