I've been using a Lumia 520 prior to my Moto G (US version). It appears both have the same AWS bands to take advantage of T Mobiles HSPA+ bands. However I've seen significantly slower rates on the Moto G than the 520. I tested a few times and am getting 4-8 mbps on the Moto G, but 12-15 mbps on the 520 using the same sim card with various tests.
I've also checked the APN settings (using the epc.tmobile.com version) and they are the same.
This is on the prepaid $30 a month scheme.
Is there anything else I'm missing in understanding the speed differences?
it is also showing in About/Phone as HSPA+ mobile network type.