In USPSA if you get a 9mm that will lock you into the Production divison. You can venture out into Limited/Limited 10 if you want but it will be at a disadvantage. There is nothing wrong with Production though. It is a budget division not a beginner division. The 5" barrel will give the maximum size gun the rules allow, and I highly recommend going the APEX route too.
I will say that having a 4.25" barrel won't hold you back, but there is a small advantage to the .75" longer barrel.
New gun, same ol' shot.