I wouldn't be so sure about plastic, cartons have plastic lining and caps. From ecological standpoint bags seem to make much more sense because they use fewer resources (just a bit of oil for plastic, no cutting of trees required), it's just a single material that can be cleanly burned afterwards and it's very efficient to transport, no space wasted for air.
Milk bags are better for the environment than cartons or jugs, even if you landfill the bags and recycle the cartons and jugs.
Bags have less plastic than jugs (thinner, softer) and cartons are difficult (eg. expensive and resource intensive) to recycle because the inner and outer layers of plastic are difficult to separate from the cardboard.
Do they? Tetrapak is "14% plastic sheets, 5% aluminium, 6% from a bio-plastic cap and 75% cardboard". Putting things in yellow bin doesn't mean that they won't be incinerated, majority of plastics either gets burned or ends up in landfills.
