I've been bowling for just over two years. I first used 14lbs because I was worried about an old High School shoulder injury, but have recently changed to 15lbs. So I will recount the differences I have noticed.
I noticed that I initially didn't have as much hook with the 15lbs as with a 14lbs. I think this is due to pure physics actually. The overall mass of a 15 lb ball is greater, so it takes more energy to get the rotation started. I suspect the inertia of the ball going down the lane has an effect on that as well as it takes more energy to make a 15lb ball "turn the corner" than a 14lb ball. Just like a lighter car can make a quicker turn, all else being equal.
Other than the hook, I found my ball speed dropped about 1 MPH according to the house monitors. However, I had too much ball speed anyway, so that was a benefit.
I was leaving a lot of 10 pins with 14lbs, and not as many as with the higher weight. That might be because of the ball speed, or I have improved as a bowler, Who knows.
Just a few thoughts from my experience, hope this helps.
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Nine in the pit with the Tenpin left standing. dooooh!!