I think the Sigg bottles are lighter than PC Nalgene bottles. The aluminum fuel bottles are not coated on the inside but the water bottles are. I think they use a flexible glass coating on the water bottles which may or may not take the heat. Sigg recommended against cooking in their anodized food boxes but I can see no reason not to....except for cosmetic reasons and possible warpage. The water bottles on the other hand do present a reasonable issue with the coating...who knows what happens to it if heated.

You could elect to use the bare aluminum fuel bottles for water however...no acidic liquids though