Hi GTLee,
Basically the difference between a soft and hard wax is this...
A soft wax is slightly easier to apply and remove and will achieve a glossier finish, but will be less durable. A hard wax will be slightly harder to apply and remove and will be longer lasting than a soft wax, but won't be quite as glossy.
However, having said that, with the Dodo waxes there really isn't much between them, as both the soft and hard waxes are easy to use, achieve great results and last for up to around three months if prep work is done correctly before the wax is applied. It's only really when you're looking at some of the top end waxes that cost mega-money that you can really notice a big difference between the soft and hard waxes.
For grey, we would recommend either of the dark colour charged waxes (Purple Haze or Blue Velvet), although, we've used Orange Crush on metallic grey a couple of times now and we've found that it does add a bit of warmth to the paint.
Hope this helps
Taryn and Jim