I don't want to sound pedantic here, but I think it's impossible to give advice on this question. People vary enormously in their sensitivity to hot/cold and their comfort zones. I have been in albergues where people were in their underwear and on top of their sleeping bags, and I was happy to be in a fleece inside my bag. I'm on the cold side, while many others may have better circulation, more natural "insulation," etc. There are lots of other variables, too -- for instance, who wins the battle of the windows and will they be open or closed; how many people are in the albergue, etc.
You probably know by now whether you're the one who always wants to turn the heat up or turn the heat down in the winter, and that will help in your decision. But I, as a "friolera," would never walk a Camino without a sleeping bag, and I usually walk in May-June. I carry the same one as Tia Valeria, a snugpak 750, and though it's often not enough to keep me warm, I don't want to carry a heavier one and I can put on extra clothes and/or hope to find a blanket.
As far as the blankets go, there have been some recent threads on this topic. I think the supply of blankets is decreasing, because of bedbugs in part. But most albergues have something you can use, though I wouldn't count on it.
Hope this helps, and sorry for the longwinded response. Buen camino, Laurie
