Depends...

You may want to provide a bit more info as to what your requirements are.

Big thread on lawn mowers here.

I have an electric (5 years old, father in laws old one) evnvironmower and petrol motormower. 

Electric mower is a tad slower to cut the grass, and doesn't have the same cutter diameter and lacks the torque of a petrol mower. 

However all of these things are due to its age of the battery I'm sure, the petrol mower is around ten years old and utterly runs rings around it. 

I'm looking about one fifty for a new battery. 

So while electrics may be awesome, petrol is definitely not going away for sheer reliability over ten plus years.