Me918bz/a Vs. Me918am?

The only difference would be the market the units are destined to be sold (signified by the BZ/A or AM suffix).

Apple offers different versions of the same product in different regions of the world, which are optimized for each area in terms of performance. Buy the model that is being offered by a store in the country where you reside......so that you will have warranty and support coverage in the event of any problems.