I've never used an RXT, but I've got an aluminum flywheel with billet insert. As I understand it:
Advantages - Lighter, less rotating mass, "free hp", quicker revving (supposedly, I don't notice)
Disadvantage - Less rotating mass can make feathering smoothly a little trickier. Heavier steel has more momentum/inertia. If the RXT isn't prone to chatter, this may be a non-issue.