With today's new self defense bullet technology, 9mm can be pretty nasty especially +P rounds.
Don't fall prey to the old stereo types about 45 acp 1 shot knock down power crap.
45acp is an awesome round, but big awesome bullets don't take the place of training, practice, shot placement, etc...
Today's 9mm rounds are more than capable, so don't worry about which is the biggest & badest.
How the heck are you going to conceal carry a FNP 45 Tactical?
That's a huge gun. Sure, it's cool, but not practical for CC.
Is this your first handgun??
9mm is better for first time gun owners. It's cheaper, less recoil, and you will need to practice with it a lot to get your skills up.
45acp is also great but more expensive to shoot, and is more for the seasoned experienced gun people IMHO.
Make sure you choose a gun you can afford to shoot because you'll want to run at least 300-500 rounds through it for break in and to get to know your gun.
After 25 years of shooting, I still love my Glock 19 9mm. I own other guns (Sigs, Rugers, Walther, Les Baer 1911, etc...) but I always seem to feel most at home with the Glock.
I did all of my training with Glocks so that's to be expected, but my Glock 19's have never let me down and I'm dead nuts accurate & fast with it.