That's a good question on the 93. I just checked etka and it lists a specific disk for the 1.9's. I'm not sure why, nor can I really tell the spline size. There is 3 different flywheels listed for that year, which leads to some confusion. As far as I knew there was single pin and dual pin flywheels. The dual pin is the early style (83-92 or so), and the single pin came on the ABA. It has a much larger inertia ring on it. I have heard something about there being a difference between german and mexican production flywheels, but I'm not sure. I'd love to hear if someone has a more clear idea of what this car uses for a clutch.
On the inertia ring - a heavier flywheel will generally allow a car to idle more smoothly, and will carry additional mass which can be good for getting the car moving. I have a Eurospec lightened flyweheel in my GTI, and you have to give it extra gas and kinda feather the clutch more to get started on a hill because the inertia just isn't there to get the car going. If you let out the clutch and try and go you can pretty much just put the gas to the floor and the car won't move. So, on the 2 vs 4 wd subaru thing I would take the guess that the 4wd subaru has a heavier flywheel to help get the car moving more easily with the heavy AWD system. All of that stuff is additional mass to get moving.