The 6906 is smaller. It will take the 5906 magazines, and there is a spacer available to cover the gap. I have been looking locally for a 5906 in excellent condition for a while now to replace my 910S. Not easy to find in this area. A dealer called me a few months ago and said that he had taken one in that was like new. When I got there I found that it was a 6906, not a 5906. I bought it anyway since it was like new. The 6906 is a really nice gun, but I prefer the larger 5906.