I love my M&Ps, and had a 9C for a while but sold it off. I still have a full-size M&P9 and two M&P45s.
For me personally, I can conceal the full-size M&P just fine and thus was deciding between keeping the M&P9C or a Glock 26 and decided on the G26 because it is a bit smaller overall than M&P9C and fits better in an ankle holster (for the few times I carry that way). Also - in a strange twist of logic - the G26's standard 10-round mags work well for me when I take my dreaded family trip to Kalifornia to see the in-laws. The M&P9c's 12-round mags are banned in Leftyland and would necessitate buying extra 10-round only mags just for those trips.
The lack of the pronounced beavertail on the compact also contributed to me liking the full-size M&P better as a shooter - but this is minor quibbling for a carry gun.
That said, nothing at all wrong with the 9C. I would not hesitate to own/carry another one if I felt I needed it in my carry arsenal (but you can only own so many guns - or so I'm told
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). The light mount up front is definitely a plus over the baby Glock, as is the extra 2 rounds of standard mag capcity. I think the M&P9C makes a better overall gun for belt carry than a G26, but G26 is a bit more universal (easier ankle carry or even pocket carry in a pinch).
Comparing M&P lineup to Glock, the M&P9c fits sorta in between the G26 and G19 and the M&P9 full size fits sorta in between the G19 and G17.
On the rust issue, had this with my M&P9 full size as well. Sent it back to S&W and they sent me a brand new slide and brand new barrel. Seems there have been some bad batches of Melonite finish on some M&Ps. Still get some spot surface rust on the slide stop and take-down lever on hot summer days with plenty of sweat. Just need to remember to oil those parts regularly. Or, do what I just did: removed the parts from all three of my M&Ps and Duracoated them as a prep for coming summer months. Should fix the problem more permanently. S&W
should Melonite these parts as well since it's a habitual problem I've heard with other M&P owners, but I guess it's a minor enough issue not to fuss with. The same parts are NOT Tenifer treated on Glocks either, but I've never had those parts rust on a Glock merely from sweat (though I've seen them rust on various torture tests when buried in rock salt, etc).