Is the forty-caliber Hi Power a better gun for protection than the 9mm Hi Power? Once again, much of that will depend upon the ability of the shooter to place the shots. The .40 S&W round is more potent than the 9mm on paper. How much this difference actually makes in real life is open to question, but I believe that it is more powerful. I do not believe that its power level will substitute for proper placement.
It seems than many users of the forty prefer the 180-gr JHP and these normally travel around 1000 ft/sec or so from a Hi Power. With the nine, the "heaviest" bullet is about 147-gr these days. With most loads, it averages about 950 to 1050 ft/sec. Velocities may be near equivalent, but the forty does have more weight and a bit larger diameter. It's almost certainly more "effective" in this weight than the 147-gr using the same or similar bullet design in 9mm.
Comparing the 9mm 147-gr to the approximately same weight Winchester 155-gr STHP, it appears that the forty "wins" again. Average chronographed velocity for this load from a Hi Power was 1164 ft/sec while Remington's 155-gr JHP averaged 1202 ft/sec. Corbon's now-discontinued 147-gr +P 9mm JHP was faster than the other 9mm rounds in this weight, but I've not tested it. The fastest I've personally seen has been the 147-gr Remington Golden Saber at 1033 ft/sec. Another company or two may be offering fast-for-9mm loads in this bullet weight but none seem to quite match velocities for slightly heavier bullets in 40-caliber.
With the 105-gr Glaser Safety Slug, the forty averages 1393 ft/sec while Corbon's 100-gr PowRball gets 1473 ft/sec in 9mm. In this instance, I believe that the 9mm is the more potent of these two specific loads.
The 9mm 80-gr Glaser Safety Slug averaged 1534 ft/sec or 141 ft/sec faster than the forty, but the bullet is also 25-gr lighter. I have not chronographed the forty-caliber PowRball.
I believe that the more defense-effective of the two Hi Powers will be the one that you shoot the most accurately at speed. If you can get accurate hits as quickly, or nearly so, with the larger caliber, that's the one to use. If not, I'd go with the 9mm.
A police officer I know was hit in the lower torso with a forty-caliber 180-gr JHP. The wound was very serious and he spent much time in the hospital. He was not immediately incapacitated. I saw the recovered bullet and it did expand.
Ballistics aside, I personally prefer the "feel" of the 9mm Hi Power. No doubt this is due to decades of using Hi Powers before the heavier 40-caliber version came along. Just because this is true for me does not mean that it must be so for everyone else. Other folks report perferring the handling qualities of the 40-Hi Power. You decide what is right for you.
Placement remains the key determinant in "stopping power." In other words, it is my opinion that the one you shoot best will be the more effective, be it 9mm or .40 S&W.