We went electric many years ago because the adverts claimed they were better at keeping plaque down - they were right. Neither my wife nor I require regular descaling now.
I think the other argument is that electric are better for your gums in that they massage them; an article in the Daily Mail the other week claimed that over-zealous brushing with a manual brush could cause the gums to recede.
As spath said, just moving the brush across your teeth and gums is the recommended way to remove plaque and help prevent gum disease.
By the way, unlike spath, we always take ours on holiday.
It reminds me of the old joke about the man who went to the dentist for a checkup. The dentist said his teeth were fine but his gums would have to come out.