What is the age and stage of your child? Before choosing a cup, make sure that it is a cup recommended for your child's age and stage. You don't want to introduce a baby-oriented cup to a toddler. The toddler may latch on to a cup that isn't safe or appropriate for his or her stage. For example, you should only use a soft-spout bottle with a baby who hasn't developed strong teeth yet. Older babies and toddlers can and will chew through these lids. If you give a baby a cup that is too advanced, the baby may be frustrated and less willing to try out different cups.
What are your parental priorities? If you know that a leaky cup is a deal breaker, avoid any cups that aren't advertised as -- or that reviews don't say are -- truly leakproof. Babies, toddlers and kids get attached to their cups, and you may end up stuck with something that doesn't work for you. By the same token, if you know you don't want your child sucking from a sippy cup, skip this phase altogether and look for a straw cup or free-flow cup.
What the experts are saying? New research and information emerges constantly -- with new recommendations about best eating/drinking practices for kids, new information on product safety and recall information when product-safety issues come up.
Be prepared to buy more than one cup for your baby. Regardless of how well-reviewed a product is, your baby is ultimately the chief reviewer. It is a good idea to buy two or three cups to try. Once you and your baby have a winner, you can stock up. This same logic will follow for toddlers and kids. Before buying an arsenal, make sure that it's tried out by the user first -- and it's likely it will be used more than once.
For such widely available products, prices vary greatly from store to store and website to website. In the grand scheme of things, sippy cups aren't very expensive -- the best stainless-steel models that can last practically a lifetime are no more than about $15. This makes it a product that leaves room for experimentation without being a big hit to the wallet.