We've done a couple of tests of the 300x600, and it's not worked so far. CTR is slightly better than 160x600, but CPC is way down. We've not stuck with it. Others might be having different experiences, though.
My 160x600 did better. The 300x600 was just average, and I hated the white space left because it doesn't collapse when they don't fill it with ads (specially on an iPhone or other mobile device; it's even worse)