Should I use 14/2 Romex or Armored Cable between plaster attic ceiling and exterior roof?
The National Electrical Code requires you to maintain 1 ¼" spacing back from the edges of framing members. 


  *300.4 Protection Against Physical Damage. Where sub-
  ject to physical damage, conductors, raceways, and cables
  shall be protected.
  
  (A) Cables and Raceways Through Wood Members.
  
  (1) Bored Holes. In both exposed and concealed locations,
  where a cable or raceway-type wiring method is installed
  through bored holes in joists, rafters, or wood members,
  holes shall be bored so that the edge of the hole is not less
  than 32 mm (1 ¼") from the nearest edge of the wood
  member. Where this distance cannot be maintained, the
  cable or raceway shall be protected from penetration by
  screws or nails by a steel plate(s) or bushing(s), at least
  1.6 mm (1/16" in.) thick, and of appropriate length and width
  installed to cover the area of the wiring.
  
  Exception No. 1: Steel plates shall not be required to
  protect rigid metal conduit, intermediate metal conduit,
  rigid nonmetallic conduit, or electrical metallic tubing.
  
  Exception No. 2: A listed and marked steel plate less than
  1.6 mm (1/16" in.) thick that provides equal or better protec-
  tion against nail or screw penetration shall be permitted.


So if you drill through the center of your 2x6" rafters you are complying with the NEC. 

However, if you are worried about future damage to your wiring from roofers, you could use armored cable. But if a responsible, licensed, insured roofer is hired they should use the proper length nails and there shouldn't be any threat to you cables.

If you really want to overbuild it use rigid pipe and wire and rest easy when the roofers come.