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Windsor Fire & Rescue Services says

There are two types of technologies used in smoke alarms to detect the presence of smoke or the products of combustion. Smoke alarms will employ one or both of these types of technologies. Each type of detector has its advantages and disadvantages.

When properly installed and maintained, both types of alarms alert you to a fire and save lives. As in all things relating to your family’s safety, buy the highest quality smoke alarm your budget will allow. Do not select a detector based solely on its low cost.

Ionization smoke alarms activate more quickly for fast, flaming fires with little visible smoke.

Ionization smoke alarms use a small amount of radioactive material that ionizes the air between two electrically charged plates, causing a current to flow between the plates. When smoke enters the chamber, it changes the flow of current, which is detected and activates the alarm.

Advantages:

Cheaper than other types of smoke alarms
Very good with fast flaming fires with little visible smoke
Suitable for general use
Less prone to false alarms due to dust and steam

Disadvantages:

Very susceptible to nuisance alarms due to cooking
May be slow to respond to slow smouldering fires
Contain radioactive material

Photoelectric smoke alarms are particularly more responsive to smouldering fires and the dense smoke given off by foam-filled furnishings.

A photoelectric (optical) smoke alarm “sees” the smoke. Smoke entering the sensing chamber reflects light onto the sensor, which triggers the alarm. Because large particles have much more surface area than small particles, a photoelectric smoke alarm is more sensitive to the large smoke particles produced in a smouldering fire.

Advantages:

Good for smouldering fires and dense smoke
Not as prone to cooking nuisance alarms
Contain no radioactive material
Suitable for general use

Disadvantages:

Prone to nuisance alarms from dust and insects – must be kept clean
More expensive

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