T-mobile has always been big on speed. I'm sure that this isn't the speed that their LTE will be and 95% of the time, these reports are based off of a single user's experience...
I was in Chicago in December, where T-Mobile's HSPA 1900 is all over the place, and my iPhone 4s & my wife's 4s constantly got 12+ down. I used my 2GB of data in 2 days, tethering. Much faster than AT&T's HSPA 1900... Unfortunately, it's only in certain parts of the area I live in, but building out fairly rapidly.
I have seen the AT&T speeds of 50+ Mbps, but only at launch. I haven't seen them much recently. It has slowed down to ~20 (still not bad for mobile data). Hopefully T-mobile will show AT&T up!