Left to right:
M&P9C 12-round magazine with flat baseplate,
M&P9C 12-round magazine with finger rest baseplate,
M&P9 17-round magazine,
M&P9 17-round magazine with X-Grip sleeve,
X-Grip sleeve
As for choosing the M&P9C or M&P40C, if it were me and I was happy with the M&P40 I'd lean towards the M&P40C to be able to use the full-size mags in the compact and to avoid having another caliber to purchase (assuming you don't already have 9mms) ... $0.02
:-)