In reply to ericinbristol:
I haven't used the Mammut Genesis for a fair few years now, but for a long time they were my benchmark as far as double ropes went.
Whilst I've used a variety of 8mm ropes over the years, including the Beal Ice Line and the Mammut Phoenix, I wouldn't recommend them if rock is your main focus. The decrease in diameter definitely has a knock-on effect to the durability, but most noticeable was the stretch, which is pretty substantial. This has (on more than one occasion) led to various people I know getting injured, as the falls they've taken have been significantly further than they'd expected. If rock isn't your focus, and winter is, or you just don't intend to fall, then it's still worth being aware of, but less of an issue.
In light of the above, I've tended to favour something in/around the 8.2 to 8.4mm mark in recent years, as that feels like you get the best of both worlds: it's a bit lighter than 8.5mm, and feels a bit stretchier, but not too stretchy.
Ironically the various ropes I've been using have either been discontinued (DMM Migrant 8.2mm) or are really tricky to get hold of and/or really expensive when you do (Sterling Duetto 8.4mm). We've got the Edelrid Starling Protect 8.2mm coming in to review soon and I'd be happy to keep you posted with any first impressions.