I personally believe T-mobile's prepaid is a better option than postpaid. Your fee is exactly $30. The other's will have taxes. If you buy refills in Minnesota you will pay sales tax on the refill, so it will cost you something like $32.25. If you happen to have a Target Red Card, you can buy the refills there and get 5% off for using the Red Card, so it comes closer to a 2.5% markup.
You can also buy online from a place like callingmart.com They are totally legitimate. They used to offer slight discounts, but now have a 2% markup as I believe T-mobile cut the commission they used to pay resellers. So it costs $30.60 and there is no sales tax charged. They have some sort of rewards program that mitigates this up charge if you regularly buy from them. They also offer both emailed codes and direct top ups. I would buy an emailed code the first time and use it online when you register your SIM and choose your plan. Then you can do the direct top ups later. You can even set them up to top you up regularly. This is better than choosing to have Tmobile top you up regularly as Tmobile will just top you up with the exact amount for your plan and you won't have any credit remaining if you go over your minutes....I usually leave about a month's credit ahead on my balance. Also if you have Tmobile top you up, they will charge sales tax in MN.
The cheapest but a bit riskier way to buy credit is to search for "Tmobile Refill" on ebay. You need to be careful though. There used be be great deals there, but not so much anymore. It's rumored that a lot of the codes for sale there used to be stolen. As the prices have risen dramatically, I think Tmobile was successful at cracking down on the criminals that were selling there. I'm guessing the remaining ones are people who have a legitimate way to get Tmobile codes (may own a convenience store or something like that). Most want you to give your number and they apply the credit directly. I have used those. Some want you to give your account PIN as well. I have never used those as it seems sketchy to me, especially as it is possible to add credit without having a PIN number.
I used to really want tethering so I could use my iPad with my phone. But I actually got a Tmobile SIM for it as well and they give 200MB free monthly. Which is more than enough for the few times I take it out of the house. That's why, even though tethering is now allowed, I haven't really had to use it much.
I am a huge advocate for T-mobile....thank goodness they finally got legitimate access to the iPhone.
The only "catch" to these rates is that they are not subsidizing a phone. So you will be spending at least $650 for a new iPhone when you decide to upgrade. The savings more than pay that back. Also prepaid plans aren't eligible for T-mobile's interest free financing plans, so you will have to pay up front. Don't buy your phone from T-mobile, but rather from Apple. If they don't change their policy getting a "device only" phone will be unlocked instantly. If you buy from T-mobile you will need to go through them to get it unlocked after 60 days or something like that...too much hassle. Also you have the freedom to buy an ATT, T-mobile, or Verizon model device only and all will work perfectly. ATT/T-mobile versions are the exact same phone with same bands. Verizon phone is the same but adds CDMA capability. Verizon would probably have best resale possibility as it could be used on any of the 3 carriers. Don't buy a Sprint one under any circumstances!
Probably too much info, but I did tons of homework on this when I switched and I'm more than happy to share my experiences!