Hi Renatab and welcome to the forum. To start with I'll say I've been quite successful detecting around the Northern Sydney area, it's a great passtime and can earn you a few dollars and other valuables. On the other hand gold prospecting/detecting can be very very difficult and frustrating, don't let that put you off though as it can be a great experience to find a gold nugget, very rewarding feeling when you find one.
To be totally honest I would not select either of those 2 detectors for gold detecting as they will struggle on our highly mineralised soils, the Xterra 705 is a very good detector but it stuggles on wet beach sand, though for parks and dry sand it is excellent. I would not recommend the 705 for gold either. But for a coin hunter it is very good
The Fisher detector I have absolutely no experience with so I can't compare the 2.
One new detector that is getting good reviews is the Makro Gold Racer, Nenad here on the forum has found a couple of pieces of gold with his and recommends them as a good budget detector for find shallow gold. Link to Nenad's video test of the Gold Racer. The gold racer would probably be good for the beach and parks also.
https://www.prospectingaustralia.com/forum/viewtopic.php?id=16502
Dedicated gold detectors (PI, pulse induction) are usually very expensive and quite difficult to operate for a novice but there is one that you can get under $4000 and that is the Minelab SDC2300, it is very easy to operate and does indeed find gold. There are older Pulse Induction gold detectors that are cheaper but they are quite difficuilt to operate without lessons.
If you have any questions please feel free to ask, hope we can help you select a detector that meets your needs. Everyone is quite friendly on here and there are some very experienced gold prospectors here also.
I would not buy from ebay or gumtree either as there are a lot of fake detectors from China floating around, when you have made a selection members here can point you towards reputable dealers.