general questions about unlocked samsung phones

I travel back and forth between Canada and USA often so I was thinking of buying an unlocked phone that will work on both countries.

I was thinking of buying SAMSUNG CAPTIVATE GALAXY S i897 that was originally manufactured with AT&T carrier but now has been unlocked.

1. Do you guys think I can use that phone for both AT&T and Rogers? (of course after inserting each of their SIM cards) They are both GSM band right?

2. Is the unlock for this specific phone permanent? or is it temporary like iphones and gets locked again after updating/restoring the phone?

3. I was thinking of using the phones WITHOUT the data plans but only with prepaid minute/texting plans and just use Wifi for internet. Would I have problem using the phone that way? Because I know many carriers will not provide contracts for smartphones without the data plans.

4. Anything else do you guys think I should make sure to check before purchasing?

Any inputs will be greatly appreciated
Thank you for your generous help.

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