The big-and-slow v. small-and-fast debate will go on as long as firearms are carried for self-defense. That debate centers on the effectiveness of the round, more so than over-penetration.
As for over-penetration, 9mm FMJ's have a reputation for over-penetrating, but I'm not at all sure it's deserved. However, one of the benefits of a good hollow point round is a decreased chance of penetrating the target. A good 9mm JHP round isn't likely to fully penetrate a target, assuming you get a solid hit, i.e. not what the old western movies called "a flesh wound." Stephen Camp's excellent web site has some good reports, tests and photos of various ammo, including hollow points.
Which side of the debate to I fall on? .45 ACP of course!
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Chas.