I carried a 6906, 6946 and a 5946 as a LEO for most of the past 15 years of my career. They have their good and bad points. I am a S&W fan so my comments are based on experience not bias. The 69 series has an aluminum alloy frame. If you buy the weapon NIB this is usually not a problem. If you buy a PD trade-in, make sure you throughly inspect the frame for stress fractures. I have seen this time and time again in the 69 series but remember that a personally owned weapon is probably taken care of much better than a "duty weapon". They are easily concealed and light weight and will handle moderate use of +p ammo. I always preferred the 6906 becasue the follow up shots were much quicker than the 6946 becasue of the TDA design. If you opt for a 6906, I would suggest that you get the newest version that you can, being the series with the rounded trigger guard and possibly the black trigger. The older model 6906's with the squared trigger guard have a flat wide trigger. The later versions with the rounded trigger guard have a much smoother trigger. During extended range sessions you will probably come to like the later versions better.
IMHO the 6906 is probably one of the better CCW handguns around. I daily carry a Glock 19 as a duty gun but I still miss my 6906.
Well, I don't have that much experience with the breed compared to estimber, but I've owned both a 6906 and a 6946. Very good guns. I also preferred the rounded trigger guards. While I've never been big on DAO guns w/o second-strike capability, I never once needed a second strike with the 6946 - darn good gun.
I went real easy on the +P and +P+ use in them (though you might look for Fastbolt's posts on the subject on the S&W Forum and Glock Talk - he's a police officer and S&W armorer, and I recall that he uses the hot stuff), though I did use it and never had a problem.
I managed a gun store for two years in the late '80s, and never once saw a stress fracture in a 69-series frame. However, that's only two years of experience with a lot of guns in general, not estimber's 15 years of experience with these pistols specifically. I've examined a lot of police trade-ins in these models and found them all to be great, but I would certainly take estimber's advice seriously.
FWIW, CDNN and Summit Gun Broker have good trade-in prices on these guns right now.
I have owned a 6906 for many years and even used it for a season of IPSC matches. Excellent trigger and sights (Novaks) and good accuracy for a stock gun. Extremely reliable; the only malfunctions I experienced were from underpowered reloads. Long term reliability was another problem altogether: at 15 thousand rounds the extractor sheared off and at 35 thousand rounds the trigger bar broke. Both problems were fixed by S&W free; great customer support. This did give me some worries about carrying a gun that was shot a lot and prone to failure, so I bought another. One for practice and one to carry.