The question is not so much how often you fly United, but more how often you fly on any Star Alliance airline. You might want to think a bit about how likely you are to end up on Lufthansa, Air NZ, and so on before submitting those miles. That said, AC isn't a bad place for them in general.

The other thing you may want to think about is how much *A flying you're likely to do in the future. If it's more than 25K status miles per year (each airline has a slightly different way of figuring these), you'll be some sort of elite*. Status with another alliance member, as you probably know from SkyTeam, will get you some benefits - early boarding, priority check-in, lounge access - but not others, such as upgrades, priority phone lines and additional award options. Those apply only to the specific airline on which you have status. If that much *A flying is a possibility, you might want to consider that in your choice as well.

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*If you thought Delta's four elite levels are a lot, would you believe AC has five?