I have a GTI 1500 though it isn't my primary beach detector, it does find stuff. Depending if you are hunting saltwater beaches or fresh, a lot depends on if you are in wet saltwater sand or dry. In the wet salt you might need the salt elimination setting. If you run into black sand you will benefit by turning down the sens. to make the detector stable. In the dry sand you should have no trouble reaching down about 8 inches on a quarter with the 8 inch coil. The double D Garrett offers is deeper but heavier too. There is always a trade off.
Make sure you overlap the sweeps and don't disc out pull tabs or you'll miss 90% of the gold rings. Lay a thin gold ring down on the sand and make sure you can read it.
Good Luck,
Sandman