Think they're actually called aluminium fibre!
Personally I'd go copper, but it's still just a gasket so doubt it really matters which ones you choose, they're probably a similar price anyway!
Make sure you give the area a good scrape out and clean where the gaskets sit, a good dollop of grease will keep them in place when refitting.
My exhaust was blowing a bit on my first MOT and it turned out that the previous owner had put the new gaskets on top of the old ones!