I have to agree with Doc about the GMT, as with most VLF machines from what I've read etc, mineralised ground is not the best places to use them. We thought about getting a VLF detector but with alot of gold area's the ground is too mineralised for them to work well, with light mineralisation I think they work ok but any more and they struggle, but thats only going by what I've read from alot of other detector users.
While the TDI Pro Oz is a great machine & would be nice to have, the SPP is still good & can find gold, its just not as powerful & is more basic, also easier to use. The discrimination feature of VLF's would be good but after a good bit of use you should be able to tell by the tone what might be under the coil but some things sound like gold so not really sure how easy it would be to tell.
I don't mind digging, never know what you might find & even if it doesn't sound like a gold tone it could be something else worth finding, like old coins, old war stuff etc. The good thing about the SPP is that it can use a good range of coils, not as many as the TDI or as big, but really, how many coils does a person need & by the sounds of it, the Minor Johns & Razorback coils have you pretty well covered.
At $1545 the SPP is hard to pass up, its not much more than the GMT & its light, so easy to use for a while which I like because of my back & feet.
One day I would like the TDI but it might be a while before I can get one, well unless I find a few good nuggets,haha.