Petrol mowers are the most popular choice for larger lawn areas and as a measure we generally recommend petrol on any any garden that is larger than the size of a tennis court. For areas smaller than this there are a number of pro's and cons to consider.
Petrol mowers - the plus points:
Generally petrol mowers have a larger cut size starting from 40 cm (15") upwards.
Grass box is generally larger on a petrol mower at 40 litres plus.
Petrol engines are more powerful and work better in long or wet grass.
No trailing cable to get wrapped around things in the garden or cut through.
Self propelled option readily available on most models to take the weight of the machine.
Petrol mowers - the down sides:
Generally heavier weight with models ranging from about 21 kgs (46lbs)
Usually require 2-3 pulls to start the petrol engine.
Need to buy and store petrol.
Require servicing every 2-3 years to keep the engine operating at its best.
Electric mowers - the plus points:
Lighter than petrol mowers usually at about 12-15 kgs (26-33 lbs)
Smaller cut widths to suit smaller gardens usually from 30 cm (12") to 40 cm (15")
No pulling to start the motor - just hold the switch.
Lower servicing costs and much simpler.
Electric mowers - the down sides:
Trailing cable can be restrictive and potentially gets cut through.
Not recommended for use in damp or wet conditions.
Generally smaller collector size.
There is no right and wrong answer to this as every garden and gardener is individual. Hopefully the pointers above will help you to make an informed choice.