Model C6906 support band 700mhz, C6903 do not support that band. The only problem which you can face with c6903 is that in some places you the connection will drop or you will not have it (LTE). C6903 will work without problems in US/CANADA but if you don't mind about that band then go for it.
LTE 800 / 850 / 900 / 1700 / 1800 / 1900 / 2100 / 2600 - C6903
LTE 700 / 850 / 900/ 1700 / 1900 / 2100 / 2600 - C6906
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My Evolution: Sony Xperia Z1 - UB + 4.3 Rooted + Deodexed, HTC HD2, HTC Diamond 2, HTC HD Blackstone, Nokia 7070 Prism, Qtek 9100, Nokia 3100, Motorola T720i, Nokia 3310, Sony Ericson PF 768 (Blue)
How Get Root on Sony Xperia Z1 with Locked Bootloader and the Latest Android 4.4