This seems quite a good summary of 4WD systems:
https://www.carthrottle.com/post/wvmekv7/
Give me Torsen all day, any day, personally...
And I agree about swapping/mixing, but only in pairs on the same axle - though I currently have matched Michelin PA4s on my S8, I have Michelin PA4s as front and Pirelli Sottogeros as rears on My Model S