40 s&w is not a "necked down" 10mm. The case is shorter but the bullet diameter is the same. Bullet weights for standard loads are too different, as is velocity. There individual effectiveness is based on two different theories: heavy and slow vs. lighter and faster. As it relates to guns themselves, magazine capacity and physical size are considerations. One fills a role just as well as the other, based on preference for a certain caliber more than anything. I would think you would have to shoot several people with both calibers then analyze the shootings. There are numerous reports written based on exactly this standard. I think if you study them there wouldn't be a huge difference in effectiveness, at least not enough to discredit one or the other.