Tmobile prepaid plan on iPhone 4?

I have a straight talk pre paid card, but my uncle told me that straight talk is no good and to use the tmobile prepaid plan. I was looking online and it said that once the sim card is shipped, i pop in the sim and it'll take me to a web browser to pick the plan I want. I want the $70 unlimited plan but there's no card for it while on wal mart there's a $50 pre paid card for it but it's limited on data… So I was wondering how this whole pre paid thing works. Is it like straight talk where you could buy the prepaid card whenever you want or is it a continual credit card payment of $70 a month. Because I may not want to spend that much every month. So let's say I want to buy $70 one month, but the next month I want the $50 plan. Can it happen? I'm really confused with their plan! Please explain how this whole thing works

I have a straight talk pre paid card, but my uncle told me that straight talk is no good and to use the tmobile prepaid plan. I was looking online and it said that once the sim card is shipped, i pop in the sim and it'll take me to a web browser to pick the plan I want. I want the $70 unlimited plan but there's no card for it while on wal mart there's a $50 pre paid card for it but it's limited on data… So I was wondering how this whole pre paid thing works. Is it like straight talk where you could buy the prepaid card whenever you want or is it a continual credit card payment of $70 a month.
--- you get billed every month like a normal Sprint/Verizon/ATT/Tmobiel customer. Pay by credit/debit or check

Because I may not want to spend that much every month. So let's say I want to buy $70 one month, but the next month I want the $50 plan. Can it happen?
--- if you want to continuous call Tmobile and change your rate plan every freakin month and prorate your bill/voice//text/data plans, then sure. More power to you

I'm really confused with their plan! Please explain how this whole thing works
--- you have spent the majority of your life on Prepaid. With postpaid, there are no little walmart or gas station calling cards. There are bills. Monthly Bills. You no pay = you no have service. Also, you no use the entire plan = oh well.

Think of it this way… For many years, you got your pizza from a shop by the slice… Well Pizza hut doesn't do By The Slice. If you only eat 3 slices, Pizza hut will not refund you the let over cost… Buy the $9.99 large pizza and deal with it OR stick with your old pizza shop… Get the drift?