My Camino partner and I are very different on this front. I always bring a silk liner, while she always brings a down sleeping bag. We're both usually pretty happy. But, based on my experience, there is some variation between routes.
On the Francés, there are lots of blankets available, particularly in the places most susceptible to colder weather (esp. higher elevation). As such, I know that I have those as a backup, in case the weather turns. (I have walked this in May-August.)
On the Norte/Primitivo, I found that nights were colder than on the Frances. Whereas I never had nights on the Frances where I would wake up feeling chilly, I definitely had those on the Norte. Blankets are not as common. Now, I could--and did--solve the problem by putting on my warm layer and hopping back in the sleep sheet. But, my body generally runs hot; other students in my group had a couple of somewhat uncomfortable nights. The ideal might be two silk liners, as a sleeping bag might still be too much on those nights, but to this point that has struck me as an unjustifiable expense. (I've walked this route June-September.)
On the Via Francigena, my friend spends at least half of her nights lying on top of her sleeping bag. I do the same with the liner. It's hot! (I've walked this route June-September.)
We don't need to talk about Norway...
Anyway, the point probably is that there isn't a perfect answer for the Norte/Primitivo. It's really important to know not just what your own comfort level is, but also what kind of effect a poor night's sleep will have on you the next day. I'm happy to trade a few chilly nights for the joy of not having a sleeping bag in my pack, but others may not feel the same. Try sleeping with the windows open at home, using only the sheet, and see how you feel!
Dave
