I would not go with kingdra, because its part water.
Magmortar has a base special attack stack of 125.
Electivire has a base attack stack of 125.
And they are pretty much equals, I'll say what I can but really it would be easier to say if I knew other pokemon you planned on using.
Here is what I would use for both.
Magmortar: Modest nature
Flamethrower
Sunny Day
Solarbeam
HP water
You can immediatly set sunny day up.
With sunny day flamethrower can do some wicked STAB damage, and if someone throws a water on you solar beam has it covered., with hp water to use his high special attack, and cover agaisn't ground AND rock, so he has no weaknesses not covered.
Electivire: Adamant
Thunder Punch
Brick Break
Earthquake
Light Screen
He pretty much covers what he can in three moves, but light screen is cool to have still. Not as much coverage as magmortar but he makes a pretty good high speed physical sweeper.
I would personally go with magmortar who covers piplups weakness to grass, and has coverage over any type that comes to battle him, sorry if this was kind of drawn out and hope this helps :D
