If I want to get to one place to another in the water as fast as possible than I prefer Sharpedo. However, if I'm going to be hunting for a certain mon, fishing, or looking for an item, then I prefer Lapras more because of the more relaxed controls.
I barely touched Lapras so Sharpedo is an instant win for me, nothing like speeding along the water and destroying rocks like nothing.
I too like Sharpedo because its faster, you encounter less Pokémon on it when I need to go somewhere. Though I do like Lapras just for GP since I used on on my original Pokémon Crystal Version
I don't really bother with Lapras as much anymore because I'm super impatient and just want to get to point A to point B asap and Lapras is too slow for me. :< So go Sharpedo!!
Sharpedo scares away fish in bubbling spots, so I tend to use Lapras, which doesn't as long as you go slowly. I've always thought Sharpedo looked a little silly too.