The ATR has a big, thick, honkin' blade with a deep hollow sabre grind. The stretch has a thinner, flat ground blade perfect for slicing and cutting.
For regular use, keep the steriods in your gun and get the slicer (Stretch) for your pocket.
You might want to consider the clip as well. The ATR has a very different kind of clip than the Stretch. The clip on the ATR is big and bulky, the Stretch has the standard spyderco clip. I never liked the ATR or Lil' Temp clips, but YMMV.
"Chance favors the prepared mind" - Louis Pasteur