Where there's smoke, there's fire - which is exactly why you need a dual sensor smoke detector. As you probably know, ionic detectors respond to flaming fires, like the ones caused when a candle ignites a piece of paper or a towel. Photoelectric smoke alarms, on the other hand, detect smoldering sooner. Considering smoldering and flaming fires happen with similar frequency, a dual sensor can literally save your life.
And yes - go ahead and hardwire those detectors. Hardwired detectors have backup electricity that assures the detectors will go off even if your house loses power. Furthermore, they're usually interconnected, so if one goes off, the others do, too.
My advice: Install a detector in every room of your house to further improve its chances of detecting a fire early. And make sure the detector you go with features an Underwriters Laboratories - or "UL" - label. This ensures its been tested and certified to meet the most recent standards.