I own both. I shoot a lot more 9mm than I do .38special or .357mag. I don't reload. I don't carry.
If I can only own one gun, a .357 is fun, but a 9mm will do what I need it to the most, and I can afford to practice with it more.
That said, I do shoot the full-sized 9mm a lot more than my M&P 9c.
For your intended purpose, either is fine. However, you didn't identify how easy it is for you to afford .38special/.357mag ammo?
For a comparison, I usually buy 9mm from Wal-Mart for $19.99 per 100. I find the .38special around $16/17 per 50, and I find the .357mag around $18-24 per 50.
Revolvers are fun. If you reload they are easy to feed. However, if you are buying factory rounds you can afford to shoot a 9mm much more often than a GP100.