Recent scholarly discussions about massive open online courses (MOOCs) highlight pedagogical and practical issues that separate MOOCs from other learning settings, especially how theories of learning translate to MOOC students' motivation, participation, and performance. What is missing from these discussions is the purpose of the MOOC. We report a comparative study of two MOOCs that differ in educational purpose, but are similar in design. Our sample consisted of 983 students in a professional development MOOC, and 648 students in a MOOC focused on general interest. We first report differences between the two MOOCs, in terms of student demographics, achievement motivation, and participation. For each MOOC, we ran a two-stage regression analysis to determine the extent to which motivation variables (stage 1) and participation variables (stage 2) predicted performance. Patterns in demographic background and motivation differed in ways that were consistent with the MOOCs' purposes. Motivation and participation predicted performance, but this relationship differed between the two MOOCs and reflected the patterns of participation. Professional development motivation contributed to final grade in the professional development MOOC, but not the general interest MOOC. The findings have implications for how MOOC designers think about their target audience, and for students who aim for high final grades.