For me, the .40 S&W is the ideal auto pistol round. I don't mind buying used firearms, and I can often get a handgun chambered in .40 for less money than the exact same model in 9mm or .45acp, simply because .40 is less popular. That can also be true for gun parts like magazines, for the same reason.
I've found that 40 caliber reloading components usually save me money; once-fired brass is commonly cheaper than 9mm or .45 cases, and I frequently find 40 caliber bullets cheaper than 9mm or 45s also. And because I reload, I can load 'down' for recoil like a 9mm, or load 'up' for energy levels better tham 9mm, .38 Super and .45acp. I can load ammo for full-size service autos, or pocket pustols, and if I ever find a nice .40 revolver I can afford, I'll load for that, too!
Factory .40 S&W ammo tends to be more expensive than comparable 9mm, which is about the only downside I can think of. However, when something happens that causes everyone to panic & start buying ammo, I've always been able to find factory .40 ammo, long after the 9mm is sold out. Plus, I personally like multi-caliber firearms (starting years ago with Blackhawk convertibles ;>), and it's fairly easy & affordable to pick up the parts needed to convert many .40s to 9mm &/or .357 SIG, or vice versa.
One last thing to consider - expanding bullets don't always perform as intended. If Murphy's Law kicks in and your expanding bullet doesn't, you might be better off with bigger, heavier bullets than smaller, lighter slugs.
As always, IMHO, FWIW, YMMV, etc., etc.