This is my latest thought process on this:

I've never had a problem using the Hammer to get the 10pin and other similar type spare shots by reducing the revs on the ball.
If lane conditions make it difficult to control the Terror then I can always switch to the Hammer or whatever I end up with.
So I'm leaning towards a urethane/low hook potential ball to be a dual purpose spare/dry lanes ball.

Right now I only carry two balls with me, the Terror and my old 16lb. Hammer urethane.