The 40 will push a wider 180 as fast or faster, in many cases, than the 9 will push a 147. I believe this matters for little problems like the FBI shooting referenced earlier in this tread; and I am certain it matters on large attack dogs, etc. I was skeptical of the .40 S&W when it was introduced; but I've seen it work in the field for a couple of decades now and I'm convinced it's a better mousetrap.
If you choose the 40 over the 9mm, there are trade-offs such as increased recoil and costlier ammunition; but many .40 S&W autos can easily be converted to 9mm- but the reverse is not the case. And yes you will give up a few rounds of capacity with the .40, in like-size guns. For the ability to use bigger, heavier bullets it is a trade many of us are happy to make.