I have a 6906 and I like it. It takes 12rd mags but will also take the 5906 mags. I have a few 5906 mags and fitted one of them with an extended based plate to close the gap between the grip and baseplate. It feels great and looks good.
It is sort of like a "Commander" version of the 5906. It can be concealed about as easy as my 3913 if I use the 12rd mags. It was cheap as well. Mine is one of the older square trigger gaurds and I got it for less than $300 OTD. It is one of those guns that can do anything, sort of like a Glock 19.
Easy call for S&W metal semis:
9mm: 5906/6906
40S&W: 4006
45: 4506/4513
There are of course many varitions, but my bias toward TDA is apparent. IMHO some of the best semi autos ever built, and I include my iconic BHP and Colt 1911 in that statement.