Which can provide the most traction on a firm to hard surface? And which can provide the most on wet ground?

The general rule is that blades are more suited to the hard surface while studs are for wet or soft ones. It varies on position -- your front and second rows need the studs for traction in the scrum regardless of the surface. You also need to be aware that metal blades are never legal on a rugby pitch -- you have to use boots with the moulded plastic type.