I also have a Shield 9 and a SIG P320c 9 (great minds...etc!), and a year ago I was also looking for a high-capacity .45. After much research, I narrowed my choices to the M&P 45 FS, the HK USP 45, and the FNH FNX-45T.
I was impressed in my initial exposure to the M&P FS 45, at a Smith and Wesson Day at my range. I fired 5 rounds at a target set at 10 yards. The first two were slightly low and left, but the final three went straight through the bullseye. I'd never shot so well with a gun I'd never before fired. The trigger was very smooth and and much lighter than the one on my M&P 40c. Subsequently, I rented one, and put 100 rounds through it in a back-to-back-to-back comparison with the HK and FNH 45s. Apparently, the M&P 45 provided at the S&W Day was somewhat of a ringer, because it had a much better trigger than the stock rental gun. I'm guessing that the S&W Day 45 had a Performance Center (or maybe even an Apex) trigger job.
Compared to my rental M&P, both the HK and the FNH had vastly superior hammer-fired triggers when fired single-action. Subsequently, I ended up buying the FNX-45 Tactical model. It was way more expensive than the M&P, but I thought its features and performance were worth the extra money for my principal home defense weapon.
My guess is that you can build a pretty good trigger on the M&P 45 if you send it to the S&W Performance Center, or if you add an Apex trigger/sear package. It will be a lot cheaper, too. I thought about doing that, but I wasn't sure what I'd end up with, so I went with the sure thing in the FNX-45T.
BTW, be sure to rent the guns you are considering, and I suggest putting at least 100 rounds through each. Not only will you learn a lot, it's a lot of fun.
Good Luck!