I always buy my Galaxy's directly from Samsung and get the 'Unlocked' version.

Apple's phones use the same 'modem' for all carriers, Samsung lists different bands for diffrent regions/carriers. I don't know if they use different modems(hardware) or they are software locked, but only the true 'unlocked' Samsungs support all of the bands.

US 'Unlocked' Version:
2G GSM: GSM850, GSM900, DCS1800, PCS1900
2G CDMA: 800, 1900 MHz
3G UMTS, HSPA+: B1(2100), B2(1900), B4(AWS), B5(850), B8(900)
3G CDMA: CDMA2000, EV-DO: 800, 1900 MHz
4G LTE: Bands 1, 2, 3, 4, 5, 7, 8, 12, 13, 14, 18, 19, 20, 25, 26, 28, 30, 38, 39, 40, 41, 46, 48, 66, 71
5G: Bands n2, n5, n25, n41, n66, n71, n260, n261

US 'T-Mobile' Version:
2G GSM: GSM850, GSM900, DCS1800, PCS1900
3G UMTS, HSPA+: B1(2100), B2(1900), B4(AWS), B5(850), B8(900)
LTE Roaming: Bands 1, 3, 7, 8, 13, 18, 19, 20, 28, 38, 39, 40
4G LTE: Bands 2, 4, 5, 12, 25, 26, 41, 46, 48, 66, 71
5G: Bands n41, n71, n260, n261