Karen:
This is a valid question but has enough variables to make it difficult to answer. I would add to the previous post that time of year, particular Albergue's and individuals comfort levels also factor in the mix.
It will be colder in the mountains than the Meseta. Some Albergue's have more modern heating systems and blankets. Others do not. We all have different comfort levels. Some like it cold others hot. Some like the windows open in any weather, some do not etc.etc.
I understand from your post that you are looking for guidance on the purchase of a sleeping bag. While I can give you my personal opinion of what works for me it might be totally wrong for you.
Sleeping bag weight might also be a factor in your decision.
I recommend you go to an outdoor store, like REI, and speak to an associate. Explain what you are going to do and provide them with factors like time of year, different environments you will be walking in, your personal comfort zone temperature wise and any concerns you have about weight etc.
Hope this helps.
Joe