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Should I use a bank or a credit union for my savings account?
Your instructor's numbers do not seem to have any basis in current reality. At this page you can see a comparison of interest rates offered by banks and credit unions.  In the most recent table for June 2014, banks paid an average interest rate of 0.12 percent on savings accounts, while credit unions paid an average of 0.13 percent.   If you look back further, you will see that interest rates paid by banks and credit unions are generally comparable.  Credit union rates tend to be a little bit higher, but certainly not 7 times higher. The last time any financial institution paid as much as 15% on a savings account would probably be the early 1980s.  You can see here a historical chart of the "prime rate" for lending.  Savings account rates (at either banks or credit unions) would typically be lower. (This is based on the US, in accordance with your tag.  Interest rates in other places, especially developing countries with less stable currencies, can be dramatically different.)