Can I use my old sim card in an unlocked iPhone 4s or 5?

I'm thinking about buying an unlocked iPhone on the Apple website. If I did, could I use my old sim/contract/plan in the iPhone? My current plan is the family plan with 550 minutes for $60 and unlimited text messaging for $20 on one line. I like my plan, so I just wanted to see if it is possible to take my old sim and insert it into an unlocked iPhone. This way, I could avoid all data plan costs because I have Wi-Fi almost 90% of the time around me!

Would that be like illegal or anything? Is it possible? Also, which one is better? IPhone 4s or 5?

Yes of course you can, it's not illegal! Le 4s is like the 5, the 5 has got a bigger screen.

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