Which Galaxy S3 to buy for prepaid?

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http://androidforums.com – Hello This question is for Tmo's 4G unlimited data +100 minutes prepaid plan. Simply put which Galaxy S3 do I buy from Amazon or Tmo to take advantage of this plan? I just want to be sure so that everything goes smoothly. I don't need to port my number. I have a contract with them currently but intend to close it this week. Should I do this before I buy or try to activate anything? Any advice is appreciated a lot as I am obviously a rookie and new to the Android/ prepaid universe. (General)