T-Mobile Prepaid has ZERO data roaming. It simply is not allowed, even on T-Mobile "affiliates".
There is LIMITED voice roaming, varies by location.
Straight Talk is the better choice if you require roaming capabilities - data included, though not always (depends on location as well)
I wasn't aware of roaming data on Straight Talk, but after reading some info about the types of networks (PCS) used by Sprint (our current--10+ yrs-- contract provider with increasingly poorer signal for us) and T-Mobile, I ordered the Straight Talk sim. For my zip, it auto chose AT&T. That's what we got for my son's GNexus, but he lives in Orlando, FL, so every carrier seems to work for him (and he has wifi available almost everywhere especially due to his Brighthouse account). While visiting us, he seemed to show the same signal on voice, but he was able to send and receive text messages on the mobile signal inside which is something I cannot do on T-Mobile. For me, it was worth the experiment into T-Mobile; however, I just don't think it will meet my needs. I also want to determine if AT&T is good enough (backed up by wifi at home) to extend to all of our phones. If not, our two remaining Sprint contracts may have to move to Verizon contracts. Ugh!