The 2 most popular styles of home Smoke are alarms are Photoelectric or Ionization alarms
Photoelectric Smoke Alarm:
These smoke alarms use a light source aimed at a sensing chamber. When the light beam is disrupted by smoke particles it triggers the alarm. Studies conducted by the NFPA have shown that these alarms work best with “smouldering” fires that have a low flame but produce a lot of smoke.
Ionization Smoke Alarms:
These alarms use a minute amount of radioactive material (Americium-241) between two electrically charged plates which ionizes the air that passes through the unit, creating a complete path of electricity. When smoke enters the unit it bonds with the ions and disrupts the flow of electricity between the sensors, thus triggering an alarm. These alarms work best for fires before the smoldering stage, where more flame then smoke is present.
Which type of alarm you chose is entirely up to you, any smoke alarm is better than none at all.
In studies conducted by the NFPA and NIST concluded that "smoke alarms of either the ionization type or the photoelectric type consistently provided time for occupants to escape from most residential fires" Regardless of the type of Smoke Alarm, all units must meet the standards set forth by the NFPA, NIST and the UL.
NFPA: National Fire Protection Association
NIST: National Institue of Standards
UL: Underwrithers Laboratories
It is recommended to have a smoke alarm of each type on every level of your home, but this doesn't mean that you have to purchase 2 different smoke alarms, many manufacturers’ offer dual photoelectric and ionization alarms.