For real gold prospecting you need a ground balanced all metal mode, and the patience to put up with hot rocks. For casual gold prospecting using a discriminator to knock out the hot rocks, a continuous-time discriminator ("analog") is generally superior to a sampled type discriminator ("digital with target ID") of the same basic sensitivity. However the BH lineup doesn't offer any continuous-time discriminators with sensitivity comparable to the midrange target ID machines without going all the way to the Platinum which I don't believe is marketed in Australia. (The Platinum has an excellent continuous-time discriminator which acts sort of like an analog machine but which is implemented in software).
Target ID machines like the 505 and the 2200 are good coinshooters but I wouldn't recommend them for gold prospecting. Between the two, the basic performance is about the same but most people would say the 2200 has a better user interface.
If you've decided your serious about the hobby and are willing to spend a bit of money for a machine which is versatile enough to handle both coinshooting and gold prospecting well, you might want to look into our Teknetics and Fisher trademark products. In the US$450-600 range the Tek Omega and the Fisher F5 are fairly similar in performance although they have very different front panels. Between those two most would regard the F5 as better for gold prospecting because of its independently variable gain and threshold controls, but apart from that most prefer the Omega. These products have been around a couple years and have a good track record, you can read about them in the forums here. We are in the process of releasing a new hotter platform (Tek G2 and Fisher Gold Bug Pro, very similar) but since these are new you won't find much information about them on the forums yet, only a little information about some earlier versions of the platform. I think they're going to be priced in the US$600-750 range. Of course we have models you can pay a lot more for, but the machines I've mentioned will do nearly as well and some would say better.
--Dave J.