I prefer glass because the only bottles available that are 5-6 gallon are #7 plastic that contain Bisphenol-A which can leach into the water. I try to limit exposure to any plastic products that are contaminated with phalates or BPA.
A local water store is just that, a local water store. I don't know where you live, but here in Southern California there are businesses that sell highly purified water and all the accessories to consume their water. The one I buy from has reverse osmosis water that goes through seven stages of filtration. They also sell distilled water.
It's better than buying from Costco or any other store that has bottled water because I don't have to wonder if the pallet of water was left in the sun on some loading dock all day. When this happens the sunlight will leach all those plastic chemicals into the water. Not my idea of pure water.
I'm pretty freakin' picky about the water I drink.