In my own opinion,I prefer Walmart because of the suitable price and after-sale service. As we all know, the Walmart owns a lot of eyeglasses for you with different types and different materials. If you have any problems after using the eyeglasses for a while, the walmart will take the responsibility of repairing them for you. I suggest you to go to the walmart to have a look. The price there is absolutely suitable.
It is hard to tell. Personally, i prefer to buy it from Walmart vision center. prescription eyeglasses there are affordable. Maybe it is just a habit. I often buy things from walmart including food and other essential commodities. Of course, i buy my eyeglasses from walmart by the way. I once bought contact lenses from costco which recommend by my friend. But i still prefer to buy them from Walmart.