I have used both alloy and steel fluwheels on my racecars
Both come in about the same weight funnily enough

I prefer the steel
I had a big overrev on a downshift and spat the alloy one off the back of the crank
Destroyed the mounting holes on the flywheel when it bounced around on the bolts
Kinda my fault anyway
I saw one on ebay made from steel but no bolt holes done yet for the clutch... won the auction at about $31 from memory (many years ago)
Had it lightened and drilled for my Toyota clutch and never looked back

So personally...I'd go steel