I have given this a lot more thought. I was with T-Mo for over 5 years, 4 1/2 on their full service contracted plan.
I loved them for the most part as far as customer service. Usually, when I called T-Mo customer service, I got an American on the phone speaking English as a first language. In the latter part of my days with them, they would occasionally bounce me to India (sounded like that anyway), but their Customer Service was pretty darn good. Their bills were outrageous. On my plan, I had BlackBerry (grandfathered in to the $20 BlackBerry Unlimited) and an Android and a two home lines on their now defunct T-Mo at home. Plus I had the now defunct $10 a month per line VOIP on all phones. With all this, even with the great grandfathered plans, m bill was north of $200 a month. I must have been loony tunes to pay that for over 4 1/2 years. I did like the fact that when I went to Canada and/or Europe I could call or take calls from the US while on Wifi seamlessly. Also, you could pay an extra $19 a month and get "BlackBerry International" which allowed you to use your BlackBerry Internet Service (BIS) plan internationally and unlimited without fear of huge data roaming. That was nice. You can't get this with any Pre-PAY plans that I know of.
On my BlackBerry, I also had UMA Transfer to VOIP automatically. I absolutely needed this as at my home/office, I have 1 bar at best T-Mo. 2 bars Verizon. Solid bars ATT and solid bars Sprint/Virgin. Their UMA VOIP service was great the first year, but then was very spotty and dropped calls or zeroed called repeatedly. I'd be on with a client and "dead air" suddenly with the infamous lost call alert tones, usually with a client that I could not afford to drop a call with. As T-Mo stopped offering their Auto Transfer to UMA (Known then as Hotspot at home), it became useless for voice in my 3rd and 4th year no matter where I was. This was sad because it was the greatest when it first launched. Understand that this was unlike any 3rd party VOIP program. This feature AUTOMATICALLY transferred you to VOIP and transferred your cell number as well, AND the ability to text and MMS via internet from your cellular number both in and out. So, if you were say, in Paris, you could take and make calls and text and mms to the U.S. for no charge via your actual cellular number. Or, if you were cruising at 25,000 feet on a plane that had Sky-Fi, you could also take and receive text and mms from your cell number (they blocked VOIP on Sky-Fi with T-Mo UMA).
I had to switch to Virgin because I needed full bars at my home and my office - and because I could not afford $200 plus per month for phone bills. So for me, it was to save hundreds a month on my phone bill, AND to be able to use my cell in my home and my office reliably without Wifi VOIP.
For me, Pre-Pay T-Mo was not any better, of course, and actually worse as I did not have the features I had become dependent on with my old contracted T-Mo plan. I still have a BlackBerry (purely business use) on T-MO, BUT THROUGH THE SIMPLE MOBILE MVNO. I use a 3rd party VOIP at the office and it works well enough on WiFi. It's genuine BlackBerry BIS service through Simple Mobile so it rocks for $50 a month unlimited which my company is happy to pay.
The bottom line I suppose is whatever works for YOU. Virgin happens to work great where I work and live.
If T-Mo did, I'd probably still be with them.
I think as they continue to expand, you will see more and more people going to T-Mo Pre-pay. Time will tell.