At the risk of sounding like I am defending T-Mobile, there are few differences.
1. T-Mobile hasn't been soft capping until very recently (I think about last 2months).
2. With T-Mobile, there's no contract.
3. De-prioritization happens only when the tower is congested.
4. Certain data, such as music streaming from select services, won't be affected.
5. In exchange, those on unlimited plans now get 7GB of tethered data access (2GB more than before), which will be throttled to 128 Kbps after instead of being cut oof altogether like before.
Having said that, I do think T-Mobile should offer Data Stash to unlimited plan. Those using less than 21GB per month should be able to roll unused data for 12 months.
Anyway, I look forward to tomorrow's Un-Carrier 10.0 Amped event, where T-Mobile will be improving many of previous Un-Carrier features to the next level.