Drupal Community Portal

I need to create a network of sites that share a common code base. So Site1 and Site2 and so-on are managed under one Drupal install.
They need to be separate sites with their own themes and modules. However they all also need the ability to share content. I need any node created on any of the sites to be shared, or not, with some or all of the other sites. I will have five sites to start, and 20 by the end of 2013.
I have looked into domain access and sites modules. There is so much info I'm not sure where to start. I can do this in 6 or 7. I'm leaning towards six, primarily because of module availability.
Any advice would be greatly appreciated.
Cheers
mr