• Main Odoo repository - https://github.com/odoo/odoo.git

• Odoo Nightly builds - http://nightly.odoo.com/  - for automatic update with apt-get
  

Github.com/odoo/odoo <== this should be the best, I think.

For automatic update, you can create a shell script and install a cron to run the script one per week or once per month which is at your choice.

I there any difference between nightly download and Odoo website download?