Hi there Chris, welcome to CS :)
I personally havent drunk milk from a paper carton in a long time.
When youre drinking from a plastic bottle, are you saying that youre getting a plasticy taste?
Obviously, different brands of milk will taste different, and different varieties in fat content (full cream, skim, hilo etc) will have different tastes and mouth-feel.
Personally I use A2 and I feel like the fullcream is a sweeter milk than other brands of milk I have tried. I also like Bannister Downs milk in WA (not sure where youre from so I dont know if youve tried that).
Heres a test for you:
Buy the same brand, same type of milk, one lot in the botlte and one lot in the carton.
Then get someone else to pour you a glass from each, and see if you can still pick the difference.
That way you can tell if the difference you are picking up on is the plastic, or if it is just the different taste in a different brand of milk.
Theyre my thoughts on the subject :)