Given the choice, I would absolutely go for the plumber that spends some of their time practising difficult plumbing jobs and doesn't just do routine cases. Even if they are 15 % more expensive. I honestly believe they'll be much better equipped to handle anomalies, should they occur.
Looking at it further, I would ignore a 15% difference if it came with any enticing reasonable. Any small difference in quality is certain to return me much more than 15%.
