No, but I might prefer a plumber from a company that pays their plumbers to train and work on the latest 'tech' in their down time. I might even be willing to pay a 15% premium, especially if I'm looking for a 'cutting edge' solution for my bathroom.

You really don’t want a "cutting edge" solution for your bathroom. You want a boring one that you will be able to maintain by yourself for years with standard tools from the store.

And I truly think it’s the same for your codebase.

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%.

this is such a poor analogy

I give you that, haven't spent much time on coming up with it.

I would like a competent plumber and if he does some fancy work in his spare time and gets even better than great.

But I would raise an eyebrow if beside my work he will also want to be paid for 6 hours he spent in the weekend, experimenting on his bathroom...

I wouldn't expect to have professional development overtly included in a short term contract, either. I'd expect a much higher hourly rate to pay for it though, amongst other things.

I'd expect if the plumber worked for a plumbing company they'd probably invest some time and money into skills growth, and pay for it out of the money they charge you.