Preference is really at the discretion of the user, however as a guideline, the following should assist you in making your choice.

ELECTRIC: Relatively maintenance free with very little to attend to other than regular checking of your cutting system to maintain safety and superior cutting. The torque (power), is not that of a petrol mower and it is preferable to maintain your lawn well to allow for ease of operation. Freedom of movement is also not that of a petrol unit as a result of the electric cable, which must be used as recommended to assist you in mowing efficiently. Cost of operation is however considerably lower than that of petrol lawnmowers.

PETROL: To maintain longevity of your engine, no matter how high the quality, it must be maintained and serviced regularly (such as a car), depending on mowing conditions. Always choose well branded and known engines such as Kohler and Briggs & Stratton, for superior quality and availability of spares. Unknown brands might be cheaper but could cost you more in the long run. The power of petrol engines is generally higher than electric allowing you more diverse cutting conditions  than with electric lawnmowers as well as the ability to cut and handle larger properties. Cost of high quality petrol units are more expensive than electric, but freedom of movement is greater.