Is the ATR empennage configuration considered a T-tail or a cruciform tail?
As far as technical specifications are concerned, some explanation is:

Excerpt from Wikipedia (formatting is mine):


  T-tail
  
  DESIGN: The horizontal stabilizer is mounted on top of the fin,
  creating a "T" shape when viewed from the front.
  
  PURPOSE:
  
  
  T-tails keep the stabilizers out of the engine wake, and give better pitch control.
  T-tails have a good glide ratio, and are more efficient on low speed aircraft.
  However, T-tails are more likely to enter a deep stall, and are more difficult to recover from a spin.
  
  
  T-tails must be stronger, and therefore heavier than conventional
  tails. T-tails also have a larger radar cross section.
  
  Cruciform tail
  
  DESIGN: The horizontal stabilizers are placed midway up the vertical
  stabilizer, giving the appearance of a cross when viewed from the
  front.
  
  PURPOSE: Cruciform tails are often used to keep the horizontal
  stabilizers out of the engine wake, while avoiding many of the
  disadvantages of a T-tail.


In reference to your comparison between ATR 42/72 with Boeing 727, if we take out the difference in tail heights (ATR 42: 24' 11"; ATR 72: 25' 1"; B727: 34' 0"), the position of horizontal stabilizer on vertical stabilizer is almost the same on these three airplanes. So they all match closely to a T-Tail configuration rather than a cruciform.