To my mind, your question implicates the same issues when we decide whether to carry a belt knife and/or a multitool.

Most of us would generally agree that a quality belt knife is a better knife than the knife in a quality multitool. But, the multitool offers a number of other features in a compact and convenient size. Unitaskers are generally better than multitaskers with respect to the unitasker's specialty. Thus, a PLB will be better at directing rescuers to your location if needed than a smart phone. A dedicated cell phone will have better battery life than a smart phone. A GPS unit will provide better service for locating yourself and planning routes while hiking than a smart phone. On the other hand, a smart phone (e.g., iPhone or Droid Incredible, etc.) can act as a cell phone, can act as a GPS device, can text and email, can store survival applications/information (e.g., first aid manual, army field guide, flashlight, etc.), and do untold other things as well.

If I were going on a day hike near a metropolitan area, I would have no qualms about leaving the PLB behind and taking a fully charged smart phone (especially if I had a hiking buddy). If I were going on a week-long canoe trip in the boundary waters, I would bring along a PLB and probably leave the smart phone behind.