I personally prefer rectangular. WIth silk, the added incremental weight will be very small.
My biggest issue, because I am a chunky guy, is that silk liners tend to cling to you as you try to slide in and out. If you need to get out of the bag to use the toilet in the night, it is a problem.
The ideal liner IMHO would be a rectangular silk sack with a two-sided "L" shaped zip to separate fully and easily. Not finding one of these after several years of searching, I did find what I think is the next best thing.
Presently, I always bring with me on any Camino, a rectangular microfiber liner I obtained here:
www.alpsmountaineering.com
I stay in rented commercial lodging as much as I can, but occasionally need to use an albergue. That is my one major 'what if' item that I always take. It compresses into a one-gallon / three-liter ziplock bag and is always at the very bottom of my rucksack.
It has a full separating zip, is machine washable, and I even used it as a very large towel in one instance. Two such liners can be mated to form a double sleeper for a couple. Plus, you cannot beat the price. This vendor is; however, located in the US.
Hope this helps.