S&W
To me, the aluminum framed guns are significantly lighter. I prefer them in 9mm Smiths, but steel in my .45s. I carried a 5904 (think blued 5903) for a long time, the 5906 seems significantly heavier to me. Check the specs on them both, I can't recall how much difference there is right off hand. Given the same circumstances now, though, I'd probably carry the M&P, but I think there are some police trade-ins of the ones you mentioned floating around for 325-375.
As to whether they are "any good," I consider them to be as reliable as anything out there. I am not an expert, but my department issued 5906s for years, and I had a lot of chances, while training officers, to see them work. I have seen a few extractors break (work hardening?), an ejector, and a side plate break...out of many, many thousands of rounds I've seen fired downrange. Their only downside, for most folks, is the DA/SA system. I learned to shoot on it and don't mind it, but some do, so make sure that is acceptable to you.
If you don't get what you need here, go to the autopistol area of
http://smith-wessonforum.com/eve/forums/ . You'll find a lot of experience with the 3rd generation guns there, a couple of armorers and more than a couple of knowledgeable tinkerers.
