Physics weight relates to downward presure of gravity.
Friction is relative to movement over the ground plane.
A combination of these can be used to suit most needs.
I would suggest you use a test level and use a box to adjust settings to extreme high or low settings and go have a look at what happens.
I would recommend not adjusting Characters settings too severely in real game levels until you do some tests as their beaviours could become erratic or you may see other undesirable results.
I find a setting of 500 for both and not the deafults is best for Characters. Otherwsie they can get stuck on paths and so on. Some Charactes wont move at all using some scripts if physics settings are incorrect or innapropriate.
"I am and forever will be your friend"