depends on what u want to use it for really decides what you should use
give you an example...the bresiani (spelling) could not break or come close to the record with a lightened (4.Xkg flywheel) whereas when jim sent him back a stock one, he ran the record and broke it by a fair bit. thats because he couldnt get it off the line and was having trouble holding boost, boost was dropping of between gears etc.
also, jim and dan have seen more warped lightened flywheels than stocko's so that also say somthing for them.
jim recommends that if the car is for out and out track racing, go for a very light flywheel (4.Xkg) thats because u want excellent throttle control mid corner, out of the corner etc, also u dont have the factor of dropping off boost because the revs are always high, also lightens the drivetrain up...drags he says to use a stock weight or slightly thinned stock flywheel, for the reasons like the bresciani example
theres more, but that's all i remember for now