First, any type of EPIRB or PLB is better than nothing when you are faced with the unfathomable situation of falling overboard, capsizing, taking on water or any other potentially life threatening situation on the water. There are options to fit every budget. Every boater should make a personal locator beacon, preferably for each passenger, and an EPIRB for the vessel, as much of a priority as life jackets. Although EPIRBs are not required safety equipment, they could be just as instrumental as wearing a life jacket in saving your life.
Second, if your budget allows for it, the Coast Guard recommends purchasing a Category I EPIRB with an integral GPS navigation receiver that you can properly mount to your vessel. Personal EPIRBs and PLBs are available, however they are manually activated, which could present a problem if you become incapacitated and cannot flip the switch. Category I EPIRBs are automatically activated when immersed in water. The drawback with having a Category I EPIRB is that it won't do much good in a man overboard scenario unless the vessel capsizes or sinks. That is why both, if you have the budget for it, is a good way to cover most potential scenarios.
If you have a Category I EPIRB mounted to the vessel, if something should happen to the boat and you are unable to flip the switch manually, an emergency signal will still be generated automatically with the owner, vessel and vessel location information to rescue agencies. If something were to happen where you are separated from your boat, you can manually activate the switch of the personal locator beacon or EPIRB to send help to your location, increasing your chances of survival.
Remember, while an EPIRB or a PLB can add an element of safety to your boating adventures, it can't keep you afloat while you are waiting to be rescued. You should choose and wear a life jacket at all times while boating. Used together, a life jacket and an EPIRB or PLB will significantly increase your odds of survival than if you used just one or the other.
Further reading about EPIRBS and Personal Locator Beacons (PLBs):