The Fors CoRe and Fors Gold are virtually identical, but the Fors CoRe has the added COG (beach/conductive ground) mode. Both are 15 kHz, so more geared towards coins and relics, but will ping gold if it's close enough to the coil.
The Fors Gold+ (plus) is a little different. It has a few extras that enhance it's gold sniffing abilities.
1. Slightly higher operating frequency at 19 kHz
2. 2 x Smaller coils as standard - industry standard 10x5" DD coil and 5" round DD coil
3. An adjustable iSAT function, which controls the stability of the unit - this can make a bit difference in goldfield soil
The Fors Gold+ doesn't have a 3-tone mode for hunting coins (some like this), and doesn't have a COG mode.
In simple terms, the Fors Gold+ will give you a better chance on gold nuggets and jewellery, whereas the CoRe will go a little deeper on larger targets and can be used down at the beach.
Cheers, Nenad