Silk one will be smaller, and more expensive than the cotton one... I've heard they are meant to be good but I feel the cold so I've got some other liner - more knitted in feel - probably cost the same as the silk one!!. What are you wanting from a liner? to protect the bag, add warmth or act as an alternative bag if really hot?
I think it's more a comfort thing as my bag is synthetic and I ge thot it it easily..I'll probably use it more on hot nights outside thje bag
BTFB, As another btfb I recently bought an extra long silk bag from here http://www.jagbags.co.nz/products.htm who are in NZ but even so the bag arrived incredibly quickly - less than a week. They also offer harder wearing/warmer versions which is what I bought. I bought mine with a view to either increasing the warmth on my existing low tech btfb style sleeping bag or to use in isolation in hot weather. The exchange rate is such that they work out to be pretty cheap.
As others have said, silk is lighter, smaller, warmer and pricier. We made our own cotton liners from old sheets. They're still going strong after all these nights but they do take up space in our panniers. On the other hand, they were free.
Cotton will hold moisture next to your body, cooling you off. Hence the climber's adage, "cotton kills". Since it retains moisture, cotton will be harder to dry when you wash it. silk is compact, cool in hot weather, adds warmth in cold weather, and dries quickly when you wash it. It's also rather expensive.
Silk is: lighter warmer more expensive faster to dry However if it's extra warmth that you want use a poncho draped over the sleeping bag to add another season to the warmth rating. I was rather sceptical when it was suggested to me by a seasoned long distance cycle tourer. He persuaded me to buy a poncho at a closing out sale of some camping outfitter that was taken over by Millets. Tried it and found that it works a treat. It also serves as a poncho/picnic blanket, bivvy and somewhere to empty the tent onto when breaking camp to minimise lossess in long grass.
Silk Silk or a similar liner is much smaller packed, easier to wash and dry and is not a great deal more expensive, I think I picked mine up from blacks
See http://www.safariquip.co.uk/acatalog/Liners_Pillows_Sheets.html They do singles and doubles in Silk. Double is £80 though.
I got mine from here http://www.nznature.co.nz/mshop:SP:788551774:tb=10 Arrived within the week and at £19.95 for a 7 footer good value.
