If the goal is to provide companionship for the canary, it would be better to get two canaries.
If the goal is to enjoy a variety of different birds, it would be better to get two cages.
Most finches and canaries are docile birds that tolerate the presence of other birds without a lot of aggression or territorial behavior. But that doesn't mean they enjoy it that much. It can be very stressful to be housed with a bird that is larger or more active or more vocal.
In a large aviary, the birds can find their own spaces. But in a cage, even a large flight cage, options are much more limited. There is more unavoidable interaction and less ability to escape, if difficulties do occur.
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Also, be sure to carefully research any birds before trying to mix them. Some finches, like zebra finches, are assertive or territorial. They are not suitable for mixing with more docile birds, including less aggressive finches.