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.