The 9mm round is plentiful in many places, plus it is a small, powerful round, edging out the .38 Spc. It has a modest recoil and is a nice defensive round.
That said, I believe the versatility of the .357 makes it a much more desirable revolver. The .38 Spc +P also is an effective defense caliber. I don't like any round I can't just drop into a chamber.
But to each his own. If someone gave me a 9mm Speed-Six, I certainly wouldn't get rid of it; however, I would rather have the awesome power of the .357, which works against people and animals. As for .38s, they're cheap, readily available and also work well against humans. They also are available in many bullet weights, configurations and powers. I've got a .357 Speed-Six and have never found it wanting.