I ordered two Transcend 32GB UHS-1 SD cards from Amazon (currently $29.99). I think it has the best value right now. It has a speed of 85/45mb read/write. I have used Transcend CF Cards (the gold one with 600x speed) for the last 6 months on the Fuji S5 and Nikon D3, and they seem fine.
By comparison, there is a Sandisk Extreme 32GB UHS-1 (currently $32.05) but the speed is only 45mb. At the high end, there is the Sandisk Extreme Pro 32GB UHS-1 with 95mbps but it's currently $68.66 each.
I intend to set the D600's slot 2 as backup (not overflow or for JPEGs). I don't want to lose photos (which has happened to me before - once is enough).