I found this chart from Bell Labs just before I went to print. The blue is 5G; the grey LTE. They are similar in size. Larger picture after the break.)
In fact, either may prove faster on T-Mobile's network, depending. The 500 megabits comes from using LTE/LAA to grab some Wi-Fi spectrum. I don't know if 5G can also soon do LAA. if not, T-Mobile's LTE will be faster. Tmo's problem is that Neville Ray has already pushed them to the state-of-the-art. The merger will give them more customers and more spectrum, but consumer results will be little changed.
The millimeter wave 5G is what delivers the performance. T-Mobile will test that but most of the next few years will be the slower 600 MHz. They want the longer reach for more coverage. Professor Ted Rappaport gave me this explanation.
"The low band (600 MHz) systems will always be limited in capacity and are not future proof no matter what special massive Mimo is used- the antennas will be too large and the bandwidths are too small, and simply can't carry the bandwidths of mmwave-- the RF channel allocations are simply too small. The wider bandwidth channels at mmwave is the only way to carry the multi Gbps data rates that will be seen in 5G. And to simultaneously also provide backhaul between cells in the same network."
Below millimeter frequencies, usually 5G NR is little faster than LTE, apples to apples. It couldn't be otherwise without breaking the laws of Physics. LTE by one measure is 90% of the way to the Shannon Law capacity; there's just not much room to do much better. There are other differences, such as the size of guard bands, but they don't add up to much.