It depends....Are you carrying the PSK in case you loose your main backpack? If this is the case, then having both bottles gives you more options.

My PSK is alays based on the scenario that it is all the gear I would have to work with if I lost my pack.

On the other hand if you think you will never loose the pack then the SS bottle will suffice.

BTW, the SS bottle Doug mentioned. The diameter of the lid is much smaller then it looks in the photo link.
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Earth and sky, woods and fields, lakes and rivers, the mountain and the sea, are excellent schoolmasters, and teach some of us more than we can ever learn from books.

John Lubbock