I'm with earthmama. I'd look at their training to deal with trauma issues and their experience in dealing with patients with trauma. Trauma therapy can be very distinct from other forms of therapy.

Personally I'm biased towards psychologists, although I do realize that they cost a lot more, but I think they get better and broader training and have longer supervised internships where they gain real clinical experience with clients, than do MSW's.

I've seen both. My current T is a psychologist, and I saw a MSW counsellor through my workplace employee assistance program. I didn't really click with the MSW, although partly that was because it was her job to "fix" me to get me back to work as soon as possible, so she was always trying to push solutions on me, that I wasn't ready for.