I have both. I find myself prefering the Mille, even though I have two of the FI 13s.

In my experience though, you can't talk about bibshorts without factoring the right saddle into the equation. My weight ranges between 210-215Ibs, and I have found out that the perfect saddle for me is the Fizik Arione Wingflex with either the Kium or manganesse rails. The saddle, with the Mille and some chamois cream makes for good 60-65miles, which is my typical ride.