Electric equipment is in some ways more environmentally friendly generating no pollution (though the electricity had to generated somehow). Once the equipment is unplugged, there is nothing to worry about with no gasoline to store. Little maintenance is needed and there is never any issue of disposing of used engine oil - since there is no engine oil. Electric equipment is also usually - though not always - somewhat quieter.
The main disadvantage of line powered electric equipment is that it is tethered to an electric outlet by the power cord. This can become quite a nuisance after a short while. Battery powered equipment has tended to less powerful and more finicky to deal with than similar equipment powered from a line cord. And, electric mowers tend to be less powerful than similar equipment using a small gasoline engine.
Where your yard is relatively small (say, less than 50 feet to an electric outlet from the farthest point), a corded mower may be a good choice. It will be less expensive than typical battery powered mowers and most gasoline powered mowers, and virtually maintenance-free. Just make sure you use a proper outdoor heavy duty extension cord - probably one size LARGER (lower AWG wire size number) than what the manufacturer recommends. This will assure minimal loss of voltage due to its resistance - and every bit of power you have available will help! A somewhat lighter duty outdoor cord can be used for the first few feet if that makes maneuvering the mower easier. The main thing to watch out for is accidentally cutting the cord by running over it. Mowing in a back-and-forth pattern while moving away from the outlet helps. If you do cut the cord - don't panic. At most, you will need to shorten it a couple of feet and install a new socket on the end of what is left AFTER pulling the plug! If the outlet is now dead, at most you have tripped the circuit breaker or GFCI, or blown the fuse. Of course if you make a habit of this, your cord could get to be quite short. :-)
Battery powered yard equipment and power tools have improved greatly over the years. Some of the newer models are quite capable of cutting a modest size yard (e.g., 1/4 acre, manufacturers specifications may still be a bit optimistic) on one charge with ample power for moderately thick grass. But, there is quite a bit of variability in cutting performance and battery life so shopping around, consulting Consumer Reports, and making sure you get a return option if you are not satisfied are all well worth the effort - to save effort in the long run.
It is interesting, however, that quite capable battery powered tractors for example, have been around for a long, long time.
See the section: Comments on electric mowers for more information.
Having said all that, the fact of the matter is that the vast majority of lawn mowers used for modest or larger lots are gasoline powered.
Of course, if you have a postage stamp size or even a small suburban lot, a manual reel mower may be your best choice - and you get some good exercise as part of the deal as well.
Also see the comments in the Chapter "Items of Interest" on electric and manual mowers.