Miles Callahan wrote an answer about Arc'teryx Beta AR Jacket - Men's on November 3, 2011
Hey Ward, you are correct that they are very similar. The Theta AR has basically the exact same fit as the Beta AR (athletic) with minor differences. Like you said, the Theta is longer and not just an inch or two, but several. I'm only 5'6" and I have the new Beta AR (which they extended the length a bit this year) and it fits my shorter torso perfectly. The Theta has a longer hem length, a slightly taller collar and an extra small pocket on the left arm which the Beta doesn't have. Also, I'm not sure which year jacket James Jenden was trying on, but both the Beta AR and the Theta AR have very large helmet compatible hoods. If you're tall or like longer jackets go with the Theta, if you're short like me go with the Beta. Hope this helps.