Stephen Sramek wrote an answer about Arc'teryx Iridine Crew Shirt - Short-Sleeve - Men's on July 29, 2014
Here are the fit blocks, in order of largest fit to smallest fit:
Iridine Crew - Relaxed Fit, hiking/trekking, durable, cool conditions
Accelerator - Athletic Fit , running/training, midweight, cool conditions, quick drying
Motus - Trim Fit, running/training, super lightweight, warm conditions - focus on moisture management.
I like the Iridine because it's durable. I use it for hiking and mountain biking. When it comes to Trail Running I go to the Accelerator or Motus depending on the weather. The Accelerator is a nice crossover piece for cool weather hiking as it's not super trim.