Well, now I have a Penetrator as well.... Blah. I'll edit this post with the results of how that goes.
EDIT: It is really, really fun to just use both. Repeater is more useful for the first dozen waves of Hard mode and against Veteran Pikemen, but after Veteran Pikemen the Penetrator is really, really awesome. In conclusion, the Repeater is better for killing large amounts of low health enemies with no shields (or with shields at the shield wall) and the Penetrator is better against anything with some strength to it. I'm pretty sure anyone could have guessed that, though.
EDIT: It is really, really fun to just use both. Repeater is more useful for the first dozen waves of Hard mode and against Veteran Pikemen, but after Veteran Pikemen the Penetrator is really, really awesome. In conclusion, the Repeater is better for killing large amounts of low health enemies with no shields (or with shields at the shield wall) and the Penetrator is better against anything with some strength to it. I'm pretty sure anyone could have guessed that, though.