What photo management application can I migrate to that would preserve iPhoto's face detection data?
I haven't used it yet, but apparently the new Photos app will continue the face detection or 'Faces' work they've had for some time in iPhoto and Aperture: 

http://www.cheatsheet.com/technology/how-apple-is-reinventing-iphoto-with-the-new-mac-photos-app.html/?a=viewall