In promoting academic achievement, is it more effective to praise students' personal characteristics or behaviour?

For instance, when we work with young people to promote academic achievement, we're encourage to praise behaviours, such as effort, rather than personal characteristics. E.g., "Keep up the good work" is considered more effective, than "You're good at this" (Cimpian, Arce, Markman, & Dweck, 2007; Zentall & Morris, 2013).