i know in formula i've smashed several carbon extensions but my aluminium is still fine and dandy. so if that helps

Yes and No. Carbon breaks because it flexes: referred to as the reflex cycle. Bend, straight, bend, straight, etc etc and it eventually breaks.... because it is a strong yet brittle material. Kevlar for eg performs much better. That's partly why hi carbon race masts often don't last as long as a 30% carbon recreational mast.
However a carbon extension is designed not to bend, and cos it is much shorter it is less inclined to want to bend.

I have used a Chinook O-ring system SDM carbon extension in waves for 4 years, and broken 5 masts in that time. It is still fine.
As I have folded an alum extension in half at the point where the holes are drilled through, and since noticed it was a little thinner tubing than others, I'd also say you get what you pay for.

Bearing in mind a RDM extension will be stronger than an SDM, if you forced me to bet on it, I'd put my $$$$ on a good brand name carbon RDM ext than a good brand alum one.
However, if the carbon RDM was a "no brander" I'd never heard of I think I'd take the good brand aluminium one (or thickest alum one I could find)

Cheers guys - I changed my mind, is the way to go and definitely Chinook.
Tim

I have had a chinook carbon RDM extension for 2 and a half years and its still fine.