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I would go for the 4S. Im on Virgin Mobile which is Bell and love my 4S. Our upgrades are not nearly as good as the US upgrades. We usually have to wait min 2yrs to upgrade most of the time. And thats why I buy my iPhones from the Apple Store each year ;)

In the US it's generally 20 months, so it's not that much different. Don't you also get unlocked phones, or at least a carrier that's willing to unlock for you?
 
If you can afford the extra $100 or so I would get the 4S. Its an overall better device than the 4, especially since you are locked in for 3 years, you know you will have software support for a while on the 4S.
 
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