The capacity of the vacuum cleaner's dust bin or bag will essentially determine how often you'll need to empty it. If you have a large house and you don't want to be going back and forth the bin, it's worth investing in a vacuum cleaner with a bigger capacity. That said, if you have trouble lifting or manoeuvring a vacuum cleaner, bear in mind that the bigger the vacuum cleaner's capacity, the heavier it'll be.
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.
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.