Electric toothbrushes have been proven to clean better than a manual toothbrush
They have big ergonomic handles and small heads which makes them great for cleaning the difficult to reach corners of your mouth, and especially easy to use for people who struggle using their hands. “Rotating oscillating” electric toothbrushes (the ones with a small round head) have generally been found to be clinically better and preferred by patients compared with sonic toothbrushes.
Manual toothbrushes can clean almost as well if used in the right way
If you use a good technique then a manual toothbrush will clean your teeth perfectly adequately.
So… use whichever works for you
If your dentist is happy with your toothbrushing, and you usually do not need a clean when you go for your checkup, then a manual toothbrush is obviously working for you.
If on the other hand your dentist sometimes points out that your cleaning could be better, and you usually need a clean when you go for your checkup then investing in an electric toothbrush will help you to clean better.