Should you invest in the hot fast-casual chain or the struggling coffee purveyor?Chipotle Mexican Grill (CMG 0.49%) and Starbucks (SBUX 1.72%) recently saw their stocks go in opposite directions. On Aug. 13, Chipotle announced that its CEO, Brian Niccol -- who had led the fast-casual restaurant's impressive turnaround over the past six years -- would depart at the end of the month to become the new CEO of Starbucks.Chipotle's stock has declined 6% since then, and Starbucks' stock surged 23%. That price acti ...