Bank notes are just "bonds of the bank", so you would purchase goods and services with Steem Dollars. The only time you purchase Steem is if you want to speculate short term or to convert to Steem Power.

Steem Dollars are the "currency" for business transactions and tipping.

You can trade Steem Dollars for Steem on the blockchain (once Steem Dollars get created on July 4th).