Even though it is supposedly "two different systems", they still manage to interact in annoying ways.
First, you can't use the same email to sign up with the prepaid system that you used for the postpaid system. I'm not sure what people do who only have one email address.
Second, assuming you keep the same phone number when you switch from postpaid to prepaid, once you login with your second email to you prepaid account and associate that phone number with it, you can no longer login to the postpaid account.
If you try to login to the postpaid account by phone number, it knows that is no longer a T-Mobile postpaid number, so it does not work. If you try to login by email, that works, but then you get stuck at a screen asking you to provide your T-Mobile phone number (and it means T-Mobile postpaid phone number).
Frankly, if I had known this beforehand I probably would not have went with T-Mobile prepaid. I probably would have went with Mint, which is a T-Mobile MVNO. My T-Mobile prepaid plan is $15/month with 2 GB of data. If you pay for a year at a time, Mint has a $15/month plan with 4 GB of data.
The only reason I went with T-Mobile is I assumed that since I was going from T-Mobile to T-Mobile, there would be no need to change SIMs and port my number. I'd just be billed differently.
Nope, new SIM and a number port.
Mint also handles data better than T-Mobile as far as I can see.
I've got WiFi at home, and most of the places I go to when not home either have their own free WiFi or there is an Xfinity hotspot near which is free to me since I have Xfinity internet, so almost never use even a gig of cellular data.
But it is nice to have as a backup. Last summer, for example, there was an accident that messed up lines on a nearby utility pole, taking out my internet. It took a couple days for Comcast to get that fixed and I relied on my mobile phone hotspot.
As far as I can see with my T-Mobile prepaid...once I hit 2 GB in a month, that's it. There doesn't seem to be any way to purchase additional data. With Mint, you can by extra data.
In short, T-Mobile prepaid is essentially like a T-Mobile MVNO, but not a very good one (no mobile app, can't add extra data on demand), and due to their systems sharing some data with T-Mobile postpaid, switching to them is harder for T-Mobile postpaid customers than it is for customers of any other carrier.