Well, there are two other issued with the 10mm. It's a long cartidge, which means only large-frame guns can be chambered in it - it's like a .45 ACP vs 9mm Glock, the .45 model is beefier. The .40 can be chambered in a standard size 9mm frame, which in turn make the pistol lighter (this is not a meaningless factor when it's carried all day) and easier for most shooters to handle.
The other, of course, is that yes you can be trained to use a heavier-recoiling gun, but the lighter one will take less training, and therefore less time and money. Don't forget there's no such thing as a professional pistol shooter - cops / soldier / federal agents have better things spend their training time and budget on than handgun shooting.
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