My dh and I like the Sonicare. Dd prefers the Oral B because it has a smaller brush head. They're both good; it really comes down to personal preference.
I asked a dental hygienists about Water Piks and electric toothbrushes. They're both good, but they are not interchangeable. She recommended using both. The issue I have with the Water Pik is that it is quite messy to use.
Also, neither the electric toothbrush or the Water Pik eliminate the need to floss at least once a day.