For ease of use, the SDC2300. Turn it on, noise cancel, ground balance, detect. It is like a vacuum for small nuggets. Handles mineralised ground quite nicely.
The GP3500 works almost as well as the 2300 with the Sadie coil on small gold. With the other coils, it can punch a lot deeper. It is a lot heavier than the 2300, but can be more versatile. The harness & bungy cord help to alleviate that heaviness.
The 4500 I have not used, but is a step above the GP3500, offering more adjustments to finer tune to suit different areas. It too can be used with multiple coils to find smaller or deeper gold. It will be a similar weight to the 3500 in that you will need a bungy cord. More complex of the 3 detectors if you are starting out.
Others will chime in with their thoughts