A watering can sounds like such a simple device that one wonders how there can be any difficulty in picking one. But, thanks to the ingenuity of contemporary designers, there are a dozen varieties. One may suit your needs better than another.
Consider the weight. Water weighs about 8 lbs (3.6kg) per gallon (~4 liters). If the watering can itself weighs a pound that's over 12% of the total if you're dispensing a gallon. Why add the extra weight? Plastic containers today are strong, rust-proof and come in a variety of attractive styles.
They may or may not have all the features you want, though.
It can be handy to have a detachable rose (that's the part on the end of the nozzle with tiny holes through which the water pours). Sometimes you want to rain gently on the plants, other times you want to deliver a gusher in a short time. That means either having two different styles, or choosing one with a changeable rose.
It's more common to find that feature with metal watering cans, but plastic ones that provide it do exist.
Size is related to weight, and also to your ability to handle it easily. A large can may hold two or three gallons but that's 16 or 24 lbs, plus the weight of the can. That's a lot of container to carry around just to water a few houseplants. Even when filled with less water, the can may be bulky and awkward.
If you get a smaller can, and there's no rule that says you must have only one, watering those indoor plants becomes quick and easy. Having watering cans in three sizes will cover the complete range of jobs you can expect to use it for.
Small bonsai, for example, can get by on very little water. In fact, overwatering is one of the most common ways novice gardeners hurt their plant. Medium-sized plants, such as an orange tree in a pot, can use all the water you can give it, provided there's good drainage. For that a larger watering can is better.
For those large, outdoor jobs you can often use a hose. But hoses are heavy and inconvenient to drag around and reel up again. Also, if you have a large yard, there may be places the hose won't go. Having a larger watering can for those big jobs makes life easy.
You may have gardens that aren't easy to plumb for a soaker hose system. A large watering can solves that problem, too. When the rains don't accommodate your plants' needs, you just dump three gallons of water around the base of the plants. If you have good soil, that retains enough but drains properly, the larger cans will deliver plenty.
Whichever type you buy, be sure not to let water sit in for long. That encourages the growth of bacteria that can be harmful to pets, people and plants. It also gives mosquitoes a good place to breed. Letting it stand overnight, to allow for evaporation of chlorine is good, but a week is too long.