You're right, I think it's pretty hard to say. But three of the CHC's are in Arizona, with three more in the west (WA, OR, and CA). So if you want to, you'll end up at a CHC out west. And the whole point of the CHC participation is to groom future physicians for their clinics, so each clinic has an incentive to help you find a residency nearby, so they can have you come back after your residency.
That, IMHO, is one of the great benefits of SOMA. You have three years to form relationships with the people who are going to be writing your recommendations, who are going to be mentoring you as to where would be great programs to go, and who know people in those programs.
SOMA's all about keepin' it in the community!