It's a matter of preference.
M = mega (in metric) = one million
MM = two of the roman numeral M = one thousand thousand = one million.
Both are correct.
M = mega (in metric) = one million
MM = two of the roman numeral M = one thousand thousand = one million.
Both are correct.
TIL
That seems rather spurious and ambiguous, considering people generally use nK for thousand units, and mixing roman numerals now makes metric nM a collision with thousands.
Compare with the "M" in CPM, which is not the roman 'M' but the French 'Mille'.
So, "M" is sometimes ambiguous. In this case it wouldn't be, but I always use mm or MM to mean million, and bn or BN to mean billion.