I think you got a typo in your text. You ask if the 6904 and 5906 are the same gun. They ain't. One's a lot bigger (the 5906).
But the 6904 and 6906 are essentially the same gun, only one is in stainless and the other is "blued." (It ain't really blue, but it ain't "silver.")
Both have 3 and 1/2" barrel, weigh about the same, have Al alloy frames but the sights are different as are the slides -- the 6906 has stainless slide and 6904 has carbon steel.
Other than that, pretty much the same gun.
Mine 6906 ain't -- it's a bit of a frankengun: started out as 6906 with damaged sights; I replace the slide with a 3904 slide which worked fine, but I was now down a 3904; then a member here sold me a 3913 slide which fit perfectly so that's now riding on the 6906 frame and it all works just fine but it won't win any "best looking in show" awards. But for a carry gun looks ain't what it's about. Here's a pic of the original 6906, the 6906/3904 and the 3914 slide.