Evening Aussie.
This little site is a great place to start when researching a detector. I keep putting it up for people to see as hopefully more newbies will see it, have a read and be better able to choose their weapon http://www.detectorprospector.com/gold-prospecting-guides/steve-guide-gold-nugget-detectors.htm
I am not wanting to sound harsh here but saying in your post that maybe the information you have put up may help someone in the future - unfortunately the guidance of a detectorist with 5 days experience on 2 new machines is probably not the best guidance for other new people entering the field. Again, not trying to have a go or start something, just my opinion. Happy to suggest a few things about both machines. And Ive owned both and still own a GM so feel that I can help with some practical experience.
Whoever told you that the SDC was the ducks guts on super fine gold was lying to you Im sorry. It is really good on really small nuggets - especially in mineralised ground. But it has its limits on tiny, tiny gold, and super fine gold - like what you get in a gold pan - is out of the reach of all detectors. For the SDC 0.01 and 0.02 of a gram is usually no problems but much smaller and it is going to depend on the ground, the day, the sensitivity setting, etc.
Why your SDC wouldnt see your test bit of gold - Im not sure. But different detectors will see different pieces of gold in different ways. I have read of people scrubbing sizeable pieces of wiry gold over the coil of a GPX and it doesnt make a sound - and thats a $5000 detector. But the GPX in many ways is the ultimate gold detector. Every machine doesnt get it all.
The GM is a highish frequency VLF. It does not eat mineralised ground for breakfast but it will find bits a fair bit smaller than the SDC, especially with the 6 coil.
Unless you have a dodgy machine there is a major depth disadavantage by detecting in Gold Mode. All Metal Mode will detect much deeper. On the machine it shows about an approximate halving of the depth in Gold Mode and in my experience that is about accurate. An almost 2 gram piece insitu could only be seen in Gold Mode when about 3 inches from it. In All Metal Mode it banged hard at 6 inches.
I am hearing what you are saying about hot rocks in All Metal Mode. They are a pain in the a$$ if you are running sensitivity too high. In hot ground I run in All Metal Mode but only on sensitivity at 3-4. Even at sensitivity 4 the GM is still VERY sensitive to small, shallow gold and it is usually much quieter on hot rocks/ground. People trying to run AMM at sensitivity 8 in hot ground are just asking for beep fatigue.
In some places like NZ they can run AMM and manual sensitivity of 10 due to really mild mineralisation. It would be a rare place in Oz that that can happen. Check out a few of John Wilsons Treasure Talks as he discusses his insanely mild ground a lot.
In another of the Minelab documents it also discusses manual versus auto and auto + sensitivity. In my experience auto sensitivity gives me about the equivalent of manual 2 or 3 in my area. The ground/rocks I detect are just too much for it and it automatically dumbs down the machine until its smooth/quiet. Yes, auto and auto + can send the machine to higher levels of sensitivity than manual 10 achieves but I personally believe those situations are rare and you would require insanely mild ground and essentially no EMI. Remote NZ perhaps?
JPs Treasure Talk or Knowledge Base articles on the Minelab website are also good starting points in running the GM.
Most of the videos that are on YouTube showing the use of the GM are terrible displays of how to use this detector and should just be ignored. Unfortunately that is what a lot of people look at before purchasing a new machine.
I know the GM is essentially sold but in my opinion you have the better machine remaining in your possession. It is called the Blue Hoover for a reason and in mineralised ground where the coil is actually going over some gold you should have a great time with it.
As others have said though, you wouldnt have any trouble getting rid of the SDC if you wanted to go for another GM. If you did that you might also look at the Equinox although small coil choice is limited at present. Apart from gold though, the Equinox will also double as a first class coin and relic machine if you decided to do that too.
If you want another GM I apparently have to sell mine to buy the Equinox. Ill keep having that argument with the good lady and if you see a GM for sale next week you will know who won 
Heaps of research, reading and checking out much of Steve Hershbachs posts on his detectorprospector website re: the detector you may be considering is worth your while.
Best of luck with whatever you decide to do :Y:
