Sharpedo, isn't it bigger and faster? Also it has rough skin so damage each time it's hurt, plus it does 1/16th damage to the enemy depending on the damage it recieved, so if KO'd it can do good damage.
Crawdaunt is slow I think (Judging the way it looks), and looks like it's built for SP. Defense, Defense, HP, and or attack and SP. Attack, but not speed. It probably doesn't have high HP as Sharpedo or maybe it does, I don't know.
Though Sharpedo definately, they are easier to catch (In my opinion), look cooler (again, in my opinion), look built for speed, attack, and SP. attack (AGAIN in my opinion), and over all they're dex entry seems more deadly than Crawdaunt (Need I say it again?), so I vote Sharpedo!
Also Sharpedo's rough skin is a good idea since, what every fighting attack I can think of is physical and the whole dark weak against fighting would work well with rough skin.
Oh and Crawdaunt I agree in a way, Crawdaunt is a Crayfish, not a crab, but could easily be considered a Lobster.