I doubt you will see any change from T-Mobile, rather they would likely argue that, at least for most customers, they are cheaper.
The difference between the AT&T and T-Mobile plans is that the AT&T plan is only 300MB data and AT&T has overage charges if you go over. By contrast, T-Mobile gives you 500MB of high speed data and, if you exceed that amount, you are only throttled, there is no additional charge.