If you're more into autocross than track evens, the 968 is the best choice. With 951s, it's not just a matter of turbo lag but rather one of non-existant low-end torque (same with the 2.5 litre n/a only worse because of the lower compression ratio). 944S2s and 968s solved this dilemma with the 3 litre crank.

Turbos have a hard time autocrossing because the boost tends to kick in just about the time you have to brake for a turn and, once you're through the turn, it's dog-slow because of the low torque at lower revs. My '87 n/a was actually faster than many of the 951s at the 2000 Parade autocross because it didn't have those combined handicaps.

 

I don't know about anyone else but when I swapped from my 2.5L 8v to the 3.0L 16v I definitely didn't notice a drop in low end torque

 

My point exactly. S2/968=low-end torque.