Conkeldurr all the way.
Seismitoad is just... Disgusting. Its french name, "Crapustule", made it even more disgust me, since I discovered it with this horrible name (on the other hand, the name Seismitoad sounds pretty good I think). And, its awesome double-type only fit the awesome Swampert, not it.
Ok, Conkeldurr isn't great neither, but between the two I totally prefer it.