I use both, depending on the pattern I am facing.

I use 15lb. when I am bowling on a shorter or lighter volume oil pattern.  This helps me keep the speed and revs up, which creates more angle and better carry.  Also, I can move a little deeper in and be able to control the breakpoint better.

I use 16lb. when I am facing a heavy oil, or medium-heavy long oil pattern.  Since I can not generate the same entry angle as I can on a shorter or lighter pattern (on average), I like using the heavier ball to help it roll through the pocket more, with less deflection and better pin carry as opposed to the 15lb. ball.  Also, if I have a shot where the backends are weak, the 16lb. ball is my choice as it smoothes out my breakpoint and rolls up a little sooner due to the slower speed.

You need to be careful not to mess up your timing if you are going to use both weights in your arsenal.  I have developed techniques to change my setup in my approach to keep my timing in check when I need to switch back and forth from 15 to 16lb.  Good luck, and I hope this helps.

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MainePBA
"I'd rather be a conservative nut job than a liberal with no nuts and no job."