The 3rd-generation S&W pistols are superb guns, and the 3913 and 5906 are prime examples of this no-longer-produced line. For concealed carry, the 3913 is a great choice. Its light weight and slim profile make it a natural for this role. I carried one myself from time to time until I discovered the Kahr K9, which I prefer only slightly over the 3913.
The 5906 is also a great pistol, but as someone has already said, it is very heavy. At 38 ounces unloaded, it's about the weight of a full-size 1911. I have a 5906, too, but I wouldn't choose it as an everyday carry weapon. For the car or a nightstand, though, it's a good choice. So you know, your friend's 5903 is different from the 5906 in that it has a lighter alloy frame. In this configuration (in addition to S&W's "value line" 915 and 910 models), the 5903 makes a more suitable carry gun than the 5906, based purely on the fact that it weighs 9 or 10 ounces less.
Gunbroker can be a good place to shop, but I suggest that you look in local shops as well. The 5906 is more prolific, so you would have a greater chance of finding one locally. Depending on condition, expect to pay about $300-$350 for a 5906. The 5903 is harder to find, but its price should not be very different. The 3913 seems to have taken on cult status with the concealed carry crowd, so prices have gone up in recent years. Good ones now go for $400-$500, and the "value line" 908 is pretty close to that amount. Still, considering the quality of these guns, the price still reflects money well spent in my opinion.
