Along with deciding between a water bottle or hydration bladder, a poncho or a raincoat, and whether to bring trekking poles or not, the Great Sleeping Bag Issue seems to be one of the most frequently asked questions on this forum.
And like every other "x vs. y" discussion here, some folks on each side of the debate are absolutely convinced that their opinion is "definitely" the correct one - when of course the answer for each individual depends on any number of variables and personal preference.
Personally, I fall squarely within the sleep sack/bag liner camp for any trip where I will be sleeping indoors. The weight and bulk of even the lightest sleeping bag simply isn't worth it for me, whether I'm walking 100K or 1000K. A sleep sack like
this one weighs only a few ounces and takes up practically no room in one's pack. If warmth is a concern (as it likely will be during the time of your walk) you can always sleep in extra layers, which you will be carrying anyway. And for extra insurance against the cold, a lightweight fleece blanket like
this one can come in handy for other things besides sleeping in an albergue (like napping on a plane or train, as a privacy screen in shared accommodations, a ground cover for picnics, etc.) and will still keep the weight in your pack at a minimum.
Again - this my .02. Your mileage, as they say, may vary
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