How to build a Drupal webform for individual/organization contributors
Yes: you can use Webform CiviCRM to make a Contribution of behalf of an Organization -> make Contact 1: the Organization; and make Contact 2: the Individual; 

No: you can not present this as a choice on the same webform - as the Contribution will be assigned to Contact 1; So: make a screen/page ahead of the webforms - with two nice buttons: "Contribute as Individual" "Contribute as Organization" -> and then link these to two different webforms;