If you estimate that stocks A and B have long-term expected returns of say 15% and 10%, you may prefer to invest mostly in A (although some investment in B may still be indicated for diversification). The situation with colleges is a bit different, because the investments are discrete. If college A has cost of $360K over 4 years and ROI of 10% and college B has cost of $160K and ROI of 15%, you can't invest as much in college B as in college A -- you would not earn two bachelor's degrees there. So you should compare an investment of $360K in college A with an investment of $180K plus $180K in financial assets.

Another problem with ROI is that adjustments for student quality are often not done. MIT graduates earn more than the average college graduate, but they would also be earning more if they went to their state flagship universities.