[cobrashoch]

As the various answers above indicate, the answer lies with what you want to do with your car and how you drive it. Steel vs. Aluminum? Heavy vs. Light? Some other things to ponder. You ever spin the wheel of bike in your arms then tried to turn it? You have to force it don't you? Now think of a car that goes around corners fast such as a Cobra. That big ol' heavy flywheel spinning at 4000 rpms and the clutch too, resists the turn bigtime. That's why most cornering race cars have smaller and lighter flywheels/clutches. Yes, a lightweight wheel will give you faster seat of the pants throttle snap too coming out of corners. But lets look at that one.
A heavy wheel gives you what is called "torque roll on" when you shift to a different gear. This, in effect, gives you a torque boost when you shift to a higher gear. In a close ratio tranny this effect is really not much of a factor and it may even be a hinderance, but in a wide ratio tranny it is important. It effects street tractability and overall performance too.
Torque roll on is important at the dragstrip because in almost ALL cases the heavier wheel will give you a better E.T. , to a point. The rule of thumb in drag racing is 10 lbs of flywheel weight for every 100 lbs of rear wheel torque your engine produces, up to about a 50 lbs flywheel weight. After 50lbs it's too dangerous to run more flywheel weight, even with a approved shield. Torque roll on is a bigger factor for heavier cars too vs. lighter cars.
Let's see, steel vs. aluminum. A aluminum wheel gives up heat much faster than steel. So if you are shifting a lot this will help clutch fade. On the other hand a steel wheel is stronger and resists harmonics much better, but they retain heat. A aluminum wheel is a bit more ticky tac to set up right, but in the case of 99.9% of the Cobras out there I would say aluminum is the right stuff to use. For my car aluminum wasn't right, when it was a close ratio stick.
Don't worry about aftermarket aluminum flywheels, they are on 2000 horsepower, 3000 lbs cars. Your car will not be a problem I'd bet.
Overall my advice on most Cobras- use a slightly heavier aluminum flywheel unless you have a close ratio box or road race a bunch.
cobrashock