To save some money, I would recommend going with the aluminum extension. I have to say though, I wouldn't go with aluminum when considering a boom.
Regarding the masts you're talking about, you never mentioned the mast sizes. You could save a ton of money if you could use a 460cm for both sails. That might mean going for a longer extension for the 8.5, but with the money saved you could always buy a shorter extension for the 6.5.
You know, I would contact either Kevin or Matt Pritchard at their website to learn a bit more. Both of these guys will let you know exactly where to be on this.