Go with the same layout as on the Widow. The Pearl cover will give you a different look on the lane -- i.e. more length. The Horizon is a great medium oil ball, something that will fit in between your Widow and your Mercury.
Moving the pin another half an inch away from your PAP will change the ball reaction a tiny bit. The pearl cover itself will do a lot more for the ball reaction. Plus, you can always change the surface, going higher in grit, or adding more polish. Roto covers take quite well to changes.
Hope this helps.
--------------------
I am the University of Virginia's Bowling Team. After all, one All-American is all you need.