Freestyle has the best safety. See http://www.iihs.org/ratings/default.aspx and you'll notice that Mariner and the SportTrac are worse than the Freestyle. Also, Mariner and SportTrac rolls over more.
That being said, if you have cargo that needs to go into a pickup truck bed, the SportTrac is the one. You do give up gas mileage with that choice, however. Its a gas guzzler.
Also, you can't get a CVT transmission, that the Freestyle has, in the other vehicles. I love that CVT. Incredibly smoothe operation and optimally efficient. Also, the more I learn about the Duratec V6, the more I like it. They've been refining the Duratec for over 10 years now.
I'm an engineer (mechanical/aerospace/software) and I'm noticing the Freestyle is built with much more sophistication than the SportTrac, and it edges out the Mariner. The SportTrac is crude (handling, steering, ride). The SportTrac will get the Ford Explorer's independent rear suspension for the '07 model year, so that helps some, anyway.