Nardo, nice starting point. Bubbles make the rear end more substantial in my mind, more of an aggressive look from behind.
If you intend driving in any form of competition dont use steel, any impact on the arch, remember they are wide, will damage the whole remaining quarter panel and turn a slight skuff into major damage.
Fibreglass will break away and can be cheaply replaced without impacting the base panel. However, if its a road and show car go steel everytime, and close the inner arch to the new panel. The finish will be much better and last, as some have mentioned fiberglass is harder to bond and can lead to shadowing or even cracking over time. however if done 100% right I have seen figerglass last very well and be indistinguishable from steel for ever.
Cheers,
Justin P