Good choice. Tough to go wrong with a lightweight .38. Put some quality +P rounds it in for carry, and enjoy.
Side note, get you a decent pocket holster. They are cheap and they'll save your pants from getting a hole warn in them as the front sight will eventually eat through your pants if you pocket carry without one. I use a Blackhawk Size 1. It covers the barrel and stays in the pocket when you draw, plus keeps it upright and easy to grasp without adding a bunch of bulk. In fact, it breaks up the outline a bit when you're sitting down and it looks more like a wallet and less like, well, a revolver.
I love my Smith 642 AirWeight. I have played with several holster options and here are a few that work for me. 1) pocket holster- I went with the Mika. 2) belly band as silly as it sounds. 3) strong side belt- I went with an IWB but even OWB with an untucked shirt will work.
If it is a new Smith I would also recommend a competent gun smith do a trigger job on the gun. Some of the new guns have decent triggers when others are not good at all.
I've heard RAVE reviews for the size/shootability of the M&P shield series. We issue glocks at my dept, but I'm considering a .40 Shield as a vest backup... good deal thinner and supposedly shoots like a full size pistol. They are MSRPed cheap too. Wait for the prices to drop and you'll get one for cheap.
Carry however you're comfortable carrying. The opinion BS can be left out of it from most. Look for size and your ability to put rounds on target with that size. ALWAYS... for the love of God, ALWAYS have a spare magazine for ANY weapon you carry. The reason has already been said in the 9 pages of this thread (could have been settled in 2 pages probably). It's a hell of a lot easier to drop a mag and clear a malf and then load back up with another mag for a lot of different malfunctions.
Revolvers: + Size, + Reliability for close contact shots, - capacity, - reload time for MANY (need practice to be proficient)
Auto: + capacity, + reload time, - reliability for contact shots (tend to jam), - hard to find small size with decent capacity for role, but they are out there.
I think S&W j-frames are great for the role and to each their own. I've just spent a lot more time with autos, so I tend to stick with them.
If you are making the concise effort to become a citizen that carries daily I don't believe you should have this mentality at all. If you want to carry a 2nd firearm as "back up" i see no issue with that but having a 2nd gun in case i don't fell like this or that is a bad idea.
Training in clearing malfunctions etc is worth more than splitting your time between 2 firearms. And if your wardrobe or something else dictates you not carrying your ccw then I would think long and hard about your commitment.
I'm really starting to like my new LC9. It's surprisingly accurate and small to boot.
I don't agree at all with people saying you should only have 1 carry gun.
I have a 380 for pocket carry in the summer and a .40 for when I can iwb with jeans and a long sleeve. I can't iwb in the summer without printing really bad, and I refuse to sweat like a pig in heavy clothing or wear a button up short sleeve to conceal a bigger gun. I'm going to be wearing shorts and a tshirt in the summer, and I'd rather have a 380 on me than a 40 printing horribly.
And yes, I practice with, and am proficient with both.
I'm a normal citizen, not Rambo or a leo. If I have a lcp on me, that will likely be good. If I can comfortably carry a larger gun, I will, and that's all the better.