The GPX 5000 is an extremely versatile pulse induction detector that can be used for gold prospecting for both small and larger gold nuggets with depth on larger nuggets easily reaching 2 feet depending on the size of the coil. It is also an outstanding beach detector though it is not waterproof and it has a very good salt mode. It is also a fine deep relic detector since it does have iron discrimination settings when used with DD coils beyond just learning what iron responses sound like. This is a one tone modulated threshold based audio detector. It is very heavy (around 7 pounds including the external battery and an 11" coil) and requires a harness and bungee to operate for long periods of hunting. There are too many coils available for this great detector to numerate here. If you are relatively young, fit and willing to swing it, the GPX 5000 is incredible. There is a substantial learning curve for most new users.
The SDC 2300 is based on Minelab's military grade portable countermine detector line. It is fully waterproof, heavy, very simple to operate and will hit very small gold as well as the GPX with its fixed 8" coil. There is a coil adapter system made by Coiltek which lets one use other GPX, GP compatible coils. It is similar to the GPX 5000 since it is pulse induction with threshold based one tone audio, but it only has one search mode and is not feature rich like the GPX 5000 which has 8 search modes and tons of adjustability. It may require a harness. If you are buying one of these used, it received some updated hardware and battery system last summer which is an improvement over the older model. It will fold down into a shape that will easily fit into any backpack. Weight is a little over 5 lbs. Its 8" coil limits its depth capabilities. It is basically turn on, noise cancel, ground balance and go, once you know what your desired targets sound like. Learning the target audio response nuances for a simple to operate pulse induction detector like the SDC 2300 is the hardest part.
Your 705 is a decent detector especially for gold prospecting for .3 gram shallow to larger gold. Larger deeper gold is possible too but is limited by soil mineralization and the size of the 18 kHz coil to about 5". Some of the 705 stock coils are not waterproof.
The Equinox 800 due to its advanced Multi IQ simultaneous multi frequency system will handle just about any soil conditions, is waterproof and is an outstanding gold prospecting detector using its Gold modes, Park 2 and Field 2 if you accept part of or all of the iron range. It will hit much smaller gold than the 705 and will also hit larger gold much deeper. I have found multi gram nuggets in high mineralization at 5 to 7" with the 6" coil. More depth on larger gold is possible with the 11" coil if that coil is not too big for mineralization levels.
The SDC 2300 will not find smaller gold than the Equinox 800 using a 6" coil. It might find gold that the Equinox can't detect due to extreme mineralization masking. The GPX 5000 using a smaller coil in Fine Gold or Enhanced might also find smaller gold that the Equinox would miss due to mineralization masking. The GPX 5000 will definitely find deeper gold than both the SDC 2300 and the Equinox 800.
Personally, if I could only have one gold prospecting detector, I was a newbie at detector gold prospecting and I did not want to lug around a heavy detector, I would make sure I had a 6" coil for my Equinox 800 and maybe a collapsible carbon fiber shaft system for easy packing, a good pick with a rare earth magnet and a couple of plastic scoops. If the Equinox 800 is setup correctly for the ground conditions it will hit any tiny, down to .05 gram gold nuggets that are near the surface and will easily hit .3 gram and larger gold that is down to about 8" depth in bad dirt. Obtaining access to easily accessible shallower gold nugget bearing land will be your biggest problem, not your detector since the Equinox 800 has that part covered.