vernt,
I'm a dedicated Tesoro user now but I have owned and used both of those detectors. The only advantage I could see in the Dual Field was that the overall detector is lighter in weight than the Sand Shark.
I am also told that the sensitivity on the Dual Field is better set for small gold than the Sand Shark, but in my experience I have yet to see any definitive proof of that. Maybe someone can enlighten us.
The Sand Shark is extremely well built and backed by a lifetime warranty. The Dual Field I think gives you a couple of years.
The volume control on the Sand Shark is on the control panel which is a very good feature to have depending on surrounding noise. On the Dual Field, you have to open the box to reset the volume which you really don't want to do in the field.
The Sand Shark comes with either an 8" or 10" coil hardwired to the unit. I prefer the 8" as it is easier to pinpoint with and still gets insane depth. The Dual Field comes with a cumbersome 12" coil that feels like you're swinging a garbage can lid after awhile and floats like a cork in the water. You have to apply pressure to keep it submerged unless you spend even more money for an after market weight made by Anderson. To me, the coil is the Dual Field's biggest drawback by far.
There's my opinion on the matter. Hope you find it somewhat useful.