If you suffer with asthma or a dust allergy, the filtration system should be an important factor to determining which vacuum cleaner you'll buy. This is because the filtration system will determine the size and number of dust particles that'll be in the air when it is exhausted out of the vacuum.
Wattage is an important consideration because motor power alone does not guarantee better suction: it depends on the overall design of the unit. If a vacuum cleaner has a high wattage, the motor is more powerful.
A high wattage (input power) doesn't always mean better suction because the amount of this power that's turned into air watts (suction or output power) depends to some extent on the vacuum's design and efficiency.