pH of sodium metasilicate anhydrous vs pentahydrate when dissolved in water
As explained in the comment by permeakra, due to the structural differences between anhydrous and hydrated Na2SiO3, their solutions will also be intrinsically different, at least for a while.
Whether anhydrous salt will turn to hydrate or vice versa depends on your ambient humidity and temperature. Anyway, I'd not recommend leaving either of them in contact with open air, for fear they might react with CO2. Silicic acid is so weak that it can be displaced even by carbonic acid.
Well, theoretically, the information about the SiO2:Na2O ratio is already present in the chemical formula Na2SiO3 (with or without water). However, the industrial-grade product may not quite match that formula. In fact, it may be a wild mixture of different silicates, where each individual compound cannot and need not be characterized. For many large-scale applications it suffices to know just the said ratio. Not sure about your application, though.