You can't go wrong with the Mission. It was one of if not the the most popular and most versatile ball made last year. With the Mission and Gamebreaker drilled similar like I have mine, the Mission goes a bit longer and the Gamebreaker hooks a bit earlier when left in NIB surface. My Mission is drilled 50* x 4" x 70* with Pin under RF and has a P3 position weight hole and the Gamebreaker is drilled 60* x 4 3/8" x 65* with the Pin under the RF and slighted the bridge and no weight hold. The Mission goes longer with a sharper break. The Gamebreaker with NIB surface begins hooking a bit earlier. Both balls have excellent hit, pin carry and continuation thru the pin deck. Both balls will handle oil. Length of oil determines which ball I start with. Overall, the Mission is stronger for me. I would definitely get that Mission your shop has if it is a good deal.
What I have just written is how it is for me with the two balls. For someone else, it might be different, I don't know.
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"Whenever I feel the urge to exercise I lie down until the feeling passes away."
Brick