What's the appropriate cable type along exposed rafters?
Jay, I think you can use NM for this application as long as you do not cross beam. Cross beam refers to bridging between collar ties or floor joists across the bottom of said members, as opposed to drilling mid member and lacing the cable through the holes or following parameter beams and extending inward for each fixture along sides of 2X4's

My first choice would be BX or greenfield plastic coated flex raceway. EMT would be great, but maybe a little impracticable for your application.