What are the benefits/costs of an aftermarket diff?
 

I'm getting into improving the drivetrain parts on my car. I'm considering getting an aftermarket diff, such as TOMS or Cusco or Tomei. I like parts that improve the driving characteristics in a more subtle way. (I have a lightweight clutch/flywheel/pulley/driveshaft that I am super happy with the difference they make.) I've heard here and there that getting a new diff makes a big difference.

But honestly I don't exactly understand how and what a better diff would improve upon? Would it save drivetrain weight and thus gain some drivetrain power loss? How would driving feel differently?

I read a bit about 1.5 way and 2 way, but I really don't know too much about diffs lol.

TOMS diffs

Tomei/Cusco diffs

Google is your friend.