Should I use "a" as in "map" or "e" as in "pet"? Merriam-Webster says both ways are ok.
There's practically an entire field of academic literature written on what's called the Mary-marry-merry merger. In "standard" British English (i.e., what's called "Received Pronunciation"), those three words are pronounced differently and the "a" in "marry" sounds distinctly like the "a" in "map."
However, among a majority of a American English speakers (57% according to that wikipedia page, including me), all three of those words are pronounced identically with something closer to the "e" sound. Within the US, the question of whether those words are pronounced the same or differently is usually one of the ways experts will try to locate the origin of your particular dialect (the dialects that do differentiate these three words are mostly confined to a few large Northeastern cities like New York and Boston and their surrounding metro areas).
Whichever pronunciation you choose is going to depend on whether you're trying to sound more BrE or AmE.