Hello. Nopes, it is sfe to store in Milton than plastic bottle.
Its because, water in plastic bottle will come in contact with sun and plastic directly and which leads to adverse effect of plastic in water.
Whereas in Milton water bottle, there isn't any contact with sunlight so reaction won't happen and water will maintain its originality.
Hope you understood.
Still if you have any queries, then please let me know. :)
Take care. :)