Ok, just for the few who do not know, Fisher originally released a 19 kHz Gold Bug model about 1987. It was a real breakthrough design at the time with a compact control box, S-rod, and elliptical coils. The detector is a good unit but is strictly all metal (no discrimination). It has no LCD readout. Some people are confusing this old model with the new when shopping used so I thought I should mention this.
Around 2010 a number of new Gold Bug models were released by Fisher and have settled into two basic versions, the Gold Bug and the Gold Bug Pro. They differ from the old 1987 model by having an LCD readout. The standard version of either detector comes with a 5" round coil. There is a Gold Bug DP (Deep Penetrating) which is nothing more than a Gold Bug Pro with an 11" elliptical coil.
The only difference listed by Fisher between the Gold Bug and Gold Bug Pro is that the Gold Bug Pro has a manual adjustment option for the ground balance and also offers "higher sensitivity".
Both models use a "Ground Grab" button as a simple ground balance method that is quite effective. The Gold Bug Pro allows you to also manually adjust the ground balance setting up or down. The manual adjustment can be used in conjunction with or separately from the Ground Grab button.
The big question is the "higher sensitivity" claim. There are two possibilities here. First, that the Gold Bug Pro actually allows for higher gain or sensitivity levels. However, I was in marketing too long and have a more jaded thought. Manual ground balance allows for a higher degree of control that if used properly can get you more sensitivity. There is a very distinct possibility the higher sensitivity claim follows directly from the ability to manually ground balance the Gold Bug Pro. This could be tested with both units set side by side with identical ground balance settings and max gain. If the Gold Bug Pro is inherently more sensitive an air test should show it. I have not had the chance to do this my self but if somebody wants to there you go.
My opinion? I believe the Gold Bug and the Gold Bug Pro if outfitted with the same coil are basically the same detector. The only real difference is the manual ground balance option on the Gold Bug Pro. Do you need it? Not really, and especially when you consider that for $499 vs $649 that is probably all you are getting. The Ground Grab function is remarkably effective and would suit most people just fine.
I personally do like manual ground balance and so for me spending the extra money to get it was a non-issue. I do as a rule tell people that if money is not an issue get the Gold Bug Pro. But in all honesty I think the Basic Gold Bug is the real bang-for-the-buck unit. There is nothing else close to it at the $499 price point that offers full LCD readout target discrimination while in full power all metal prospect mode.
I should note that First Texas owns both Fisher and Teknetics. The Fisher Gold Bug DP (Gold Bug Pro with 11" coil) is marketed by Teknetics as the G2. The Fisher Gold Bug DP goes for $699 and the Teknetics G2 is $749. The $50 extra gets you a pistol grip rod instead of the Gold Bug S-rod and an arm strap. Nice gray paint scheme also. Really boils down to pistol grip vs S-rod, purely a personal preference thing.
Finally, I use the 5" x 10" elliptical myself and consider it to be the best all around coil for the Gold Bug. However, right now you have to get it as an accessory or as part of a two coil package. Fisher would be doing us a service to release the Gold Bug with this coil as standard on the unit.
I have had zero service issues with my Gold Bug Pro and use it hard. My Gold Bug 2 is slightly better on the tiniest of gold but the Gold Bug Pro easily outperforms the Gold Bug 2 on larger nuggets at depth. For all around nugget detecting the Gold Bug or Gold Bug Pro have a better balance of both small gold and large gold capability than the Gold Bug 2.
Hope that helps.