The plusses to a lighter flywheel is less power is used to accelerate the flywheel, so less power is wasted. The down side, RPM's drop faster between shifts.
The plusses to a steel/Iron flywheel is that although it's slower to accelerate, it adds more mass to the engine, which helps keep RPM up during launch and causes car to launch harder because of more centrifugal force. You're alot less likely to bog the engine with a heavier flywheel.
Also aluminum flywheels don't dampen engine vibration as much so the car will have a rougher feel with the engine running and more firing pulses jar the transmission and driveline, which can shorten drivetrain life slightly.
This is what i've read/heard, maybe someone else has heard otherwise.
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2005 Suzuki Hayabusa GSX1300-R
1980 Ford Thunderbird - 255 V8
ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain
hi-po Mack Truck hood emblem
1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc.
Sold 02/06/04 
1989 Mustang GT ET: 13.304@102.29 mph (5-24-03)
Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)
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