tendoboy1984 - that's because those phones are sold by T-Mobile and are designed by Nokia and Samsung (your T989 Galaxy S 2) respectively to work on T-Mobile's network (an AT&T Galaxy S 2 (i777 or i727) will NOT get 4G on T-Mobile). Like I have been trying to explain, the Moto G does not support simultaneous 1700Mhz and 2100Mhz which is required to connect to T-Mobile's 4G (not LTE) network. From the Moto G specs page
Networks
Global GSM Model:
GSM/GPRS/EDGE (850, 900, 1800, 1900 MHz)
UMTS/HSPA+ up to 21 Mbps (850, 900, 1900, 2100 MHz)
US GSM Model:
GSM/GPRS/EDGE (850, 900, 1800, 1900 MHz)
UMTS/HSPA+ up to 21 Mbps (850, 1700 (AWS), 1900 MHz)
CDMA Model:
CDMA/EVDO Rev A (850, 1900 MHz)
For comparison, here are the specs for the T-Mobile Samsung Galaxy S 2 (T989, what you have)
2G Network GSM 850 / 900 / 1800 / 1900
3G Network HSDPA 850 / 1700 / 1900 / 2100
I understand that for some people this is hard to understand, but I think I've made it pretty comprehensible.
Plugabugz - it depends on the phone and the service, T-Mobile has both LTE and HSDPA, just like AT&T.
As a side note, T-Mobile Galaxy S phones are typically the best version to get, because they will do 4G on T-Mobile and AT&T (Verizon version will also do 4G on AT&T).