I am taking a Wallaby class at my LYS so have seen a number of different yarns being "made up". I am making mine in Cascade 220,it is nice but I am not overly thrilled with it. The one I am ABSOLUTELY IN LOVE with is being made out of Berroco Pure Merino, it is just stunning. There is something about the yarn itself that fives the stitches a much different look than the Cascade.
I just orderd some Berroco Pure Merino on ebay for $4.50 a skein (7.50 at my LYS) and plan to make my next one out of that.