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?
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.
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.