For my motor, steel. I spend MUCH more time on the street than on the track. The extra weight of the steel flywheel makes for better drivability. There's allot to be said for intertia of a heavier flywheel when up/down shifting during the rigors of daily driving. The aluminum is great for spinning the motor up faster and is ideal for the strip. I guess it comes down to which you do more of or which you area you want the best performance for.
Go with a steel flywheel. As stated above with a lightweight flywheel you may have to feather the gas pedal at idle to keep it running. Plus you will make more torque with a heavier flywheel.