That's like asking how long a bit of string is really :8 ..... First thing to consider is what type of ground (deep? shallow? Hot or benign?) your going to be 'prospecting', and the 'type' of Gold known in those areas (Riverbed fines, or hillside alluvials from reefs? etc)...
The 4500 will give you a more adaptable unit over varying ground & gold types, whereas the 2300 is reputed for catching that very small/fine Gold many tend to walk over/past...
Like any activity, it's best to suit your tool to the job at hand, and without further info., it's hard to specifically recommend either over the other, yet both are competent at what they were designed for.
I have my own personal choice of the two, but that answer may not be the best solution for you and your circumstances.
Any chance of some more info. please?
Gypsy