You probably aren't going to own just one for very long if you want to practice much. When I was in your shoes some time ago I bought a .357 Magnum. However, I quickly found out I could burn through $80 of ammo at the range in one trip. I quickly ended up buying a .22LR pistol which is what I shoot more often at the range to save money. I do most of my shooting with .22 and still fire off about 20-30 rounds through the .357 each time out.
Two guns can actually be cheaper to own than one.
Like the man told me: Guns are like shoes; dress for the occasion. You wouldn't wear your sneakers to a business meeting or your flip flops to go hiking in the mountains.
9mm or .357 Mag are good choices for what you want to do, but realize that you might end up with two guns quicker than you think.