I have a .40sc and I am really happy with it. If you get the .40sc they will have a drop in barrel in the near future for the 9mm.
The .40 is going to have a little more muzzle flip and the .40 has more knockdown power. You can debate shot placement and all that jazz but I'd rather have the .40 and you can always get the drop in barrel for the 9 when it comes out if you so desire a 9. The best of both worlds I think.
About the only complaint I have about the .40sc is the recoil/guide rod system. Mine is coming unscrewed and I have to make sure to keep it tighten during range sessions. I need to call Springfield and get another one, the guide rod/spring should not come un-screwed.
If you shoot great with the 9 you should stick with it. Each person shoots different with different weapons. If it ain't broke don't fix it. Peace.