HOTSPOT is best for your scenario.You can configure UserManager [Builtin small radius server in Mikrotik] to create prepaid voucher/tickets. When the user will try to browse, he will be redirected to your page which you can customize according to your requirement. After entering Id/Password, he can use his internet, and after expiry/usage, he will be disconnected.
Some links that may help you in achieving your goal:
http://aacable.wordpress.com/2011/09/12/mikrotik-hotspot-quick-setup-guide-cli-version/
http://forum.mikrotik.com/t/sample-hotspot-page-sticky-please/23665/1
RouterOS or Routerboard, both can fulfill your requirements.
If you are going for RB, If you are talking about 20-30 users , I guess RB 750G would be more then enough for you.