Looking at the comparison on Bowlingball.com, the perfect scale is similar, so the number of boards covered should be close. Viewing the RG and Diff, the Epidemic will flare a little more, go a bit longer and have a more angular hook to the pocket than your Rival. Pretty much what the shop owner said.
Why are you replacing the Rival with another ball that provides a similar reaction? You can get it resurfaced for a lot less than the price of a new ball.
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