I've had both. There is no difference in stiffness and overall ride feel. I broke my chain ring tab on the Atlas cranks after about a year of use and am waiting to hear from Race Face about a warranty replacement. I've only had the Descendants for a few months so I don't have a huge sample size, but so far, so good. Descendants are lighter and billed as a "race" crank so my guess is that they probably are a tad weaker.
If there's nothing wrong with your Atlas cranks, I'd say it's not worth buying the Descendants unless you're trying to shave the weight (about 80 grams). I've been guilty of that a few times, I must confess
