In your case when you are running several domains with same template, you should choose the second approach. It involves putting a Content PlaceHolder
on the .master
page on the place you want to have the logo afterwards (top left corner, for instance). This is how you will be able to drop the corresponding image in the PlaceHolder depending on the domain where you have loaded this template.
As for your second question, the page structure in Sitefinity contains three main nodes as follows:
Normal page: displays its own content, editable within Sitefinity.
Page group: redirects to its first sub-page.
External page: a link to a page created outside Sitefinity
The Page Group does not have its own theme or template, as it represents several Normal Pages which are placed under one node (Group Page). Its basic function is to act like a different web site branch where you can place its own navigation controls.
Hope that this information answers your questions.
the Telerik team