
Dum-Dum?
True Dum-Dums are crosshatched, some deeply cut. They fragment into five pieces of lead if done properly. As mentioned above hollow-point rounds often fail to expand properly because of clothing, though bullets like the Golden Saber and Barnes X are designed better, and Federal HSTs are probably the best in terms of expansion (2x vrs. 1.5x diameter).
Penetration of 40 S&W hollow-point rounds ranges from 10-16 inches in bare gel (typically), while a standard round might achieve 16-18 inch.
Personally, in terms of self-defense, I believe the heavier the round the better, and naturally two holes are better than one.
OTOH, we in the US have allowed our police departments to become increasingly militarized and in the process increasingly distanced from the citizens, and paranoid to boot. The recent shooting of a teenager in Seattle because he had a cell phone in his hand is a good example.