IMHO it's a matter of personal choice.
The M&P9c's major pluses include slightly lighter weight and a slightly more carry-friendly shape with fewer external protrusions. Other M&P pluses are the interchangeable backstraps, burgeoning aftermarket- particularly holster and sight availability- and the uniform shot-to-shot trigger pull; you don't have to deal with the DA-to-SA first-shot transition. OTOH the pistol's major minus is its trigger pull quality and long reset, which I don't mind, but some shooters seem to actively hate.
The M6906's major plus is a really nice trigger: smooth DA, not-quite-tuned-1911-nice but still very good SA, and the reset against which all other DA/SA pistols are judged.
It has a standard safety and mag disconnect, which some like on a carry gun. Minuses? That DA-to-SA transition. The ambi decocker/safety levers and relatively big slide stop of the M6906 are obnoxiously obtrusive to some; the lack of safety levers is a reason why some shooters favor the less common decocker-lever M6926 or the leverless DAO M6946. There are no common and practical replacement options for the wraparound grip unit; if you don't like the ergos, that's just too bad.
M&P pistols generally lack a mag disconnect except for special-order (particularly LE) guns, but I regard mag disconnects as a matter of personal choice, and I don't want to veer into discussing their pluses and minuses here. FWIW the M6906 mag disconnect
can be removed non-destructively, BUT the procedure requires drifting out the rear sight- not everyone's cup of tea.
Minor but mandatory M6906 warning notes...
The pistol came with two types of rear sight: a vertical blade used on 1988-early 1990 production, and a slanted Novak Lo-Mount unit used from late 1990 onwards. Although the early blade-style unit isn't awful, the Novak is better because it's more carry-friendly and offers more sight radius and aftermarket options. The clincher: the two do NOT interchange readily because the slide dovetails are different sizes!
The pistol came with two basic styles of trigger guard: a somewhat unsightly square guard was used until about 1992, and rounded guards were used later (there were a couple of very subtle and largely unimportant changes to the shape). Round trigger guard pistols will usually fit in square trigger guard holsters but NOT the reverse. This makes holster selection better for the round trigger guard pistols.