The two options are not exclusive. We have supermarkets in Vancouver that sell milk in glass bottles.
Glass is cheaper if you have the infra-structure in place.
* You have the crates to move filled bottles around
* Trucks move between dairy and store, with no holding point between.
* The diary has the automatic washing equipment.
* The transport distance isn't huge.
Disposable containers make more sense:
* Haulage distances are large (takes less room)
* Not a direct connection between the bottling plant and the store
* Disposing of used containers in landfill is cheap, or there is an efficient recycling program in effect.
If you can get the industry to standardize on a single bottle of each size, then glass gets points.
Getting this established is difficult. You need a dairy that is large enough to supply a reasonable chain of stores to get the volume you need.
Here, plastic milk containers run about 30 cents more (~10%) than cardboard. Not sure why people still buy plastic.
In Alberta for decades we had a standard size 12 oz beer bottle, called the 'stubby' Every brewery used it, and on the average a bottle made 6 round trips before being lost or broken.
They stopped making this a requirement, and there are now many different beer bottles -- too many to sort effectively. Bottles are broken for cullet and made into new bottles, a process only slighly cheaper than starting from sand and lime.