Plastic containers provide the best value for storing drinking water at a large scale. The toxins are a complete non issue imo: Most houses have some sort of plastic plumbing now a days. I would bet there is a pretty good chance that the water you drink today at home or at work has probably been in contact woth plastic.

The worst case scenario with toxins from plastic leaching into your water is Cancer. Which takes decades to possibly develop, if it ever does. Most likely you'll starve to death or die of an infection before the cancer gets you. Cancer is super low on my list of acute issues post collapse.

Stainless steel is pretty much going to be the best container you can get to store water in. It is super expensive unless you have a source for cheap stainless barrels. But it is pretty much the best container material there is for food or water storage. It will cost 10-20x more then similar sized plastic containers.

Cement cisterns are another option that is extremely robust and super common in places that use cisterns.

So really the only benefit of stainless is that it may reduce the small small likely hood of cancer once a collapse hits. But you would be better off spending that money on plastic containers which would secure a larger quantity of water to prevent the more likely crisis of running out of water.

Keep in mind this is emergency water. Your not going to be drinking out of these containers your whole life.

My water storage is three tiered: 2x 55 gal food grade plastic barrels in my basement. This is my bulk emergency supply that is not intended to be moved.

3x 5 gal jerry cans rotated several times a year. This is my mobile supply that I could load into my car and leave with.

12x 2 liter pop bottles that I keep filled and frozen to take up any empty space in my chest freezer. This helps increase the thermal mass of my freezer so it will be cold longer without power and it also makes for convent ice packs for a cooler or an injury.

My resupply plan: About 700 gals of rainwater catchment off my roof.

My well which is pretty shallow and can be run off solar or a generator if I need it to be.

Lakes and streams.

At least a dozen different ways to purify and clean water: including Berkey style filters, sawyer filters, life straws, sand filters, rv water filters, boiling, iodine, bleach, pool shock and plenty of wood to boil water with.