Rental Unit: 30 year vs. 15 year mortgage

My fiance and I are looking at a rental property that would would yield about 8.4% Cash on Cash ROI based on a 30 year loan and the low end of rental income possibility. We've considered doing a 15 year loan since our incomes could support it, but then we would basically hit a 0.0% Cash on Cash ROI. Does that even make it worth it? I'd rather have the passive income, pay extra when possible, and turn a monthly profit. Am I thinking about this correctly? Other than the surface advantages (such has shortly loan, lower interest, etc), are there any other advantages? Thank you!