Swapped from a 996 BIP to a 998r but I have also ridden couple of 998s.
The later engine is a step forward IMHO, smoother and pulls harder.
Some of the differences IMHO depend what model & model year your 996 is. 2000 MY and later 996 got the 5 spoke wheels and nitrided forks.
If it has non nitrided forks (silver sliders) then you should notice a step forward on a 998s as it has nitrided forks.
I didn't find that the **8 felt any lighter than a 6, maybe a few kilos in weight according to the Specs.
There was no difference in steering effort required IMHO.
Some say they like the rawness of the 996 v 998, not me though
If it was my £££££, yep, I go for a 998s over a cooking 998 as well. Much bigger step than going from a 998s to 998r IMHO.
Good luck in finding a good 998s. You might need a big helping of patience
Ray.