"Water and foods freeze differently", according to the Penn State Extension. First, you want to freeze items as quickly as possible. The faster the freezing, the smaller the ice crystals in your food. The smaller the ice crystals, the less damage to the cell structure of the food. Remember, your freezer will go through freeze-thaw cycles. This is where the potential damage occurs. Evaporation occurs during these cycles. This is known as freezer burn. One way to protect against freezer burn is to package your food with as much of the air removed as possible. So, a freezer bag will be better than Tupperware, and a vacuum sealed bag will be even better. At the very least, with a zip-style freezer bag you can use a straw to suck out much of the air...or, even better use Archiemedes principle. In terms of convenience, bags are much more effective, not only do they take up less space, but items can be spread out in the bag, as flat as possible, making thawing faster.