IMHO, there are a few factors to consider before deciding between FMJ and JHP. Now, these are my own opinions, and unless I am carrying a good personal defense round, I always carry FMJ when available but here is my opinion on the matter:
1) Consider reliable feeding. Some autos have problems feeding anything but FMJ ball ammo, but not always. Practice with what you are gonna carry to be sure there are no hiccups, even if the stuff is $1 a round.
2) Consider the area. Where are you going to be? I live in a desert so over-penetration is rarely an issue. If I am going to be in town, I carry Hydro-Shoks a lot but not religiously. If I am going into the middle of nowhere, I can punch through my target and not worry about hitting something behind it.
3) Consider the weather. Cold weather means people will usually be wearing thick clothing and jackets. Rumors say that a hollowpoint clogged with a chunk of leather coat may not expand as desired.
4) Consider the most likely scenerio. If this is a car gun, a fire fight may break out with a few layers of metal in the way. A FMJ may help punch through without as much of an energy loss.
5) Taking all of this into account, and knowing it is hard to plan for something you want to avoid, you may try an old trick. If you get reliable feeding, stagger FMJ, JHP, FMJ, JHP, FMJ, JHP, FMJ, JHP, FMJ, JHP, FMJ, JHP, FMJ, JHP, FMJ, JHP.
Hope this helps!