I have no problem with either caliber with appropriate ammo. I have owned a 9MM Shield and carried every day since 2013. Until the ammo changed in a good way, I had no use for the caliber. When the .45 Shield was announced, my curiosity was piqued. I found no fault with the 9, but had a hard time vizualizing how the .45 could be a really good thing in the Shield platform. Never thought they couldn't do it, but after getting one to satisfy my curiosity, I became an immediate believer. I've always liked the .45ACP regardless of the projectile, but with premium ammo, it's even better.
But my needs centered around an EDC gun that was easy to carry, easy to shoot and use, was reliable to my standards (very high!), and accurate for the purpose. The platform had to fit my hands and allow me to shoot it as easily and as well as I am capable of at this time in my life. I had already determined that the 9MM would meet those standards with a reasonable amount of ammo and could be reloaded more easily and spare loads could be carried more easily than my beloved revolvers with which I had over 40 years experience. The platform also operates (draw and shoot with nothing to fumble with just like the revolvers I am so programmed to shoot.
With all this, I bought a .45 Shield. Didn't need it, but my curiosity needed massaging. It was. I was absolutely positively astounded by it. I don't say it's better than the 9, at least not for it's purpose, but it is absolutely as good. And for me, there was nothing not to like about the .45 if I could stand to shoot it in the Shield platform. I could ... in fact, I loved it! The trigger was better than my 9, the recoil was not noticeably more than the 9, shot recovery time was plenty good for predator control, and accuracy was as good or better than the 9. In fact, I shoot it a bit better than the 9. And ammo capacity? I'm used to 6 (or 5) rounds before reloading, remember? Depended on it most of my adult life! And I carry extra ammo too!
So now, I'm carrying the .45 every day, all day. But I still would not hesitate to go out and about with the 9. But it's kinda my back up EDC now. If you now own a 9MM Shield and get along with it just fine, I see no need for you to spend any more money for anything but ammo for practice. If you are curious like me, or if you just like the .45 like me, and you can afford to scratch your itch, I can give you no reason not to proceed to scratch. Not many things feel much better than scratching an itch! I predict if you are itching, scratching will bring a great big ole smile to your face! But remember, 9MM ammo is cheaper than .45, you do get an extra round or so with the 9, and you only have to stock one caliber. Me, I already had a pretty good selection of .45ACP as well as the 9MM, so I wasn't adding a new caliber to my ammo shopping list.
My wife has a 9MM Shield. We have a FS M&P 9MM with 17 +1 rounds ready to go at the head of our bed. That same gun goes on trips with us as a car gun. But I'm now carrying the .45, so we carry both calibers in loaded mags for backup ammo in the car bag. A bit more "trouble", I guess, but I am very happily carrying the Shield .45 while keeping my 9MM on standby and ready to go. So my advice is to keep your 9 and scratch your itch. If that is satisfying, then you've got two great personal protection guns. Somebody said, "Two is one, one is none." A spare ready to go is never a bad idea if you can afford it. The truth is, neither one of these fine Shields cost much in the overall scheme of things.
Sorry for the long winded post. Hope you find something here to help you make up your mind. But in the end, you are the only one who can make up your mind for yourself. Different things work for each of us. The challenge is to find out what's best for you. If you can't get good hits with it, it ain't for you!!