What is the effect of the difference between the materials used to make lightened flywheels?

Exedy make their Racing Flywheels out of solid one-piece forget chrome moly steel ( http://www.exedy.com.au/RacingFlywheel.aspx ).

Clutch Industries (PBR) RPM offered lightened flywheels from 3 different materials:
• Lightened Steel Flywheel
• Chrome Molybdenum (Chrome Molly) Flywheel
• Lightened Aluminium Flywheel
( http://ciperformance.com.au/flywheelspage.html )

Anyone know what effect the different materials have (i.e. do the steel ones last longer than the aluminium ones, but are heavier, etc?)?

aluminium is expensive and usually for racing purposes only, steel flywheels are more durable and can be lightened accordingly, a few kilos can be taken off to reduce reciprocating mass. Revs up quicker but it can affect torque. Not something u guess at, talk to a machinest and tell him what u want to do with it.

found this might explain easier :
In the quest for faster cars, many enthusiasts will have their flywheels lightened. Why? Because a lighter flywheel increases the rate of torque applied to the drive wheels. How? It increases the efficiency of the drivetrain by reducing inertia. This is important as it will accelerate you car faster than the identical engine with a heavier flywheel. Also, if you are going to lighten a flywheel, it's much more effective to remove weight from the outer edges than the interior. This is due to the "leveraging" effect that mass has from the center of rotation. Like an ice skater spinning with their arms out, and as they bring them in they spin faster - same energy, faster rotational speed.

The downside of this is that taking off from a dead start is more difficult, because there's less energy at the clutch (energy=(1/2) mass x velocity-squared (.5 x m x v^2)). Decrease the mass, increase the speed (revs). Underdrive pulleys and lighter wheels help acceleration using this same principle - no more torque at the crank, but delivered to the wheels faster. Lighter wheels have the same effect, but to a smaller degree.

I was reading about those aluminium ones, and they actually have a friction material on them, which is replaceable, so the aluminium flywheel should last forever, and you just replace the friction material. Might give them a call and ask about how they go in a street car.
Sort of similar concept to the rear sprocket that I use on my dirt bike - it's an aluminium sprocket with a steel ring gear riveted on - the maximum use of aluminium to make it as light as possible, but still using steel on the gear as you need the hard wearing surface so your not constantly changing the cog.

The i6 being a long stroke engine though will minimise the ill effect of a lighter flywheel.

Generally 4 cylinder jap engines on the other hand (having power strokes further apart and shorter strokes) can be noticably slower when taking off with a lighter flywheel due to the combined effect of their lack of torque and momentum of the lighter flywheel (or is it inertia? f**k, last physics was 4 years ago!).

Saw a 1 cyl industrial diesel engine once, has a long long stroke, but the power strokes being so far apart it required a seriously huge a** chunky flywheel.

the lighter flywheel lets your engine rev up quicker and also slows down quicker so engine braking is a little more effective. my preference would be a single untreated material one as it would be more consistent for it's usable life