I have an iphone 4 but:
1.) I have the family plan with my wife (we have unlimited data also because when they did away with it we were grandfathered in to keep it)
2.) We upgraded to ours
3.) Ours is through at&t
We was going to try a different phone but we liked our iphone 3gs's so much and were so comfortable with them that we decided to just upgrade to the 4g. Very happy with that descision too I think Waltah said verizon's data plan is slower then at&t's before but I am not sure if he actually said that or not. He is a VERY good person to ask this question too though BTW