At one point in time I owned both a 442 and a M&P340. (The 442 has since found a new home, but the M&P340 is a keeper). There are some distinct differences between the two, including the finish. The finish on the M&P340 is much harder and way more durable. The M&P340 also has more of a flat finish when compared to the matte/satin finish of the 442 IMO.
For those considering one vs. the other, you'll have to decide whether or not the differences between the guns are worth the price difference. (FWIW, my 340 was less than $600 w/ the current $50 rebate from Buds.) Here are my reasons (in order of importance to me):
Sights - The biggest factor for me was the XS sight. When you compare the sight picture between the two, the M&P340 wins hands down. It can be difficult to see the front ramp in dark or dim light situations on the 442. This was the #1 deciding factor for me. I even looked into getting the sight shaved and an XS added to the 442, but the smithing and refinishing would have put the total cost beyond the 340.
Caliber - Having the flexibility to shoot .357 was a big plus, even if I carry w/ .38 +p. If there is ever another ammo shortage or other situation where ammo may be hard to obtain, the flexibility will come in handy. Shooting .357s is a handful, but not as bad as some of the ladies would have you believe.
Quality - The fit and finish are excellent and in a different league when compared to the X42s. It's just a nicer made gun and lacking the more frequent QC issues that can arise on the X42 series.
Weight - I pocket carry on occasion and the weight savings is nice.
Other - If it's going to be carried, it's going to get worn. The finish on the 340 will hold up better. The ejector shroud is also nice - one less thing to get snagged in the pocket.