Pls advise me on the different between model A1634 &

The difference is that only the A1634 has ATT's LTE Band 30. The A1687 is good for all bands on other carriers and it will work on ATT but not on their latest Band 30. Band 30 gives better coverage in high density coverage areas (high traffic). So if are going with ATT you want the A1634. IPHONE 6sPlus. A1633 is their 6s (smaller) iPhone.