I've used the Explorer II and GT.
I was not impressed with the Ex.II.
I was very happy with the GT.
To me the added electronics and cost in the Explorer was to pay for the meter and gimmicks. But it's possible I didn't understand it and/or it was defective.
I got my GT near mint for $350 shipped. I found small silver jewelry with it that my other machines totally missed. The GT is one of the best there is for silver and ignoring nails.
Average depth on coins for the GT is like all the other VLF detectors that cost from about $200 to $1400....8 inches. Greater depth is possible under ideal conditions and with larger coils.
Some use the 15 inch WOT on the GT and claim depths of over 10 inches on coins (up to about 12 inches).
The Tesoro Silver uMax with 12x10 concentric coil is as deep as any other VLF machine. I hunt areas where snow plows pile snow and also push some dirt with the snow. This means very deep coins and rings. I regularly dig memorial cents at 10 inches deep with the Tesoro. Tesoros aren't as good on silver as the GT. Tesoros are much better on copper than the GT. Tesoros love the copper alloys and lead. They are the ideal Civil War Relic machines.
But Tesoros also love hot rocks and deep iron. With experience these can be avoided but it takes practice.
If you're primarily after silver--get a Minelab product (excellent on silver- good on copper).
If you're primarily after copper alloys/lead---get a Tesoro Silver uMax with the 12x10 concentric coil (excellent on copper - good on silver).
This is my opinion as of 8/17/08 10:12 A.M. EST (updated every 15 seconds).
Better yet get both!
This is the process I use and it works great here but maybe it's different where you live?