Ballet teachers often express a preference for leather shoes as they require the foot to work harder to stretch. The extra effort to form a pointed foot in a leather shoe develops strength in the muscles of the feet and legs. Some teachers specify a full leather sole, as opposed to a split-sole shoe, for the same reason. Before you take the time to get fitted for ballet slippers, be sure to check with your teacher whether he has a preference. Many teachers allow adults to wear canvas even if they require leather for younger students.
If you need shoes that will last a long time, canvas may be a better choice. Leather shoes stretch with wear and can lose their shape easily. Canvas shoes do not change shape much, though they may snag and form holes quicker than leather shoes. Canvas shoes are also easier to clean, since you can put them in the washing machine.
Different cuts and styles of ballet shoe flatter different types of feet. Some dancers like canvas shoes as they appear to present a better pointed shape of the foot. Other dancers prefer the look of leather. The aesthetics of different brands vary as well. Try on a few brands of both canvas and leather and see what works best for you. You will feel more confident in class if you are wearing shoes that complement the lines of your legs and feet.
If you take class at a studio solely dedicated to ballet, often times dancers will use rosin to keep the feet from slipping too much. Dance floors should be slick enough to allow you to turn, but not so slippery that you fall and injure yourself. Studios that share the floor with other dance class styles often don't allow rosin, since it can cause problems for those dancing with bare feet. If your studio does not allow rosin, a leather ballet shoe is a better choice. If your studio allows you to use rosin, you can go with canvas if you prefer.
Don't ever buy shoes that are too big and assume you will grow into them. Leather shoes should be a little tight, to allow for stretching. Canvas shoes should fit your feet perfectly, like a sock. If you are buying your first pair of ballet shoes, you may want your teacher to come with you. If this is not possible, make sure you buy your shoes at a dancewear store. The staff at a store dedicated to dance clothes and shoes will be able to help you choose shoes with the right fit. Don't buy your first pair online or at a regular shoe store.