OK. Doing some more research--lots of info out there. Seems that the difference between the T999 and i747 is that the T999 has support in the 1700 band for HSDPA, while the i747 has LTE capability. It appears that T-Mobile is migrating away from the 1700 band in favor of LTE. This chart
includes LTE coverage for a bunch of cities, including pretty much anywhere that I spend significant time.
indicates that SM should operate on LTE. So, from this info it looks like the i747 might actually be the better option--although I think that either one will work fast enough for my needs. Basically, most anything faster than EDGE is fine for me.