Take a look at the prepaid coverage map and the postpaid coverage map on their website. There is a major difference. No reason not to go postpaid since it's no contract either and you get tethering built into each plan.
It depends on your use. If you make lots of calls, I suppose postpaid can be a good option.
I don't talk too much when I'm out and about and if I want to make a lengthy call when I'm at home I use either the Magic Jack or Vonage app to use VOIP. (They will work with 3G/4G/LTE as well but quality can be unpredictable).
Therefore I opted to go with the $30 a month 100 minute, unlimited text, 5GB 4G plan. Even if I did go over some on minutes, it would come out better any postpaid plan. Also, I am kind of bummed at losing tethering, but I honestly only did it occasionally. If I really want it, I can pay $15 a month to get it back (and it wouldn't be limited to just 500MB).
As mentioned there are no taxes added on the bill, so exactly $30 is charged each month. Also, I'm able to buy refills on eBay for 10-20% off so my actual cost is $24-27 monthly. If I decide I want tethering it would be $36-41 monthly. So both are still less that any post paid plan would be with taxes.
Some say the coverage is different for prepaid and postpaid, but I've yet to be anywhere that I had no coverage at all. I think prepaid doesn't get to roam on other networks in areas that T-mobile doesn't have native coverage. If you live in a rural location or travel through them a lot, it may make the difference for you.