My concern (which is possibly unfounded) is that it would take significant investment before the productivity gain made it worth it. Anecdotally, I took a job

My concern (which is possibly unfounded) is that it would take significant investment before the productivity gain made it worth it. Anecdotally, I took a job as Rails developer some years back. I had zero experience with it before starting (Having worked mainly with PHP before), but because the environments are so similar (http is the same, gnu/linux is the same, mysql is the same, architecture and oop is the same etc. etc.), I was creating value after about three weeks and I would say that I was fully up to speed after something like 3 months. Consider that it can easily take the same time to get the business, the problem domain and your new colleagues to know, I'd say that the cost for my employer was close to nothing. Now, I'm not saying that anybody can do this - I consider myself a fairly good developer and beside PHP, I have tinkered with a broad spectrum of languages, which surely helped my transition. And the point isn't that Ruby is such an easy language that it can be learned in 3 weeks either (In fact, I think it is a rather complex language). What I take from my experience is that a good (and diverse) developer, with a motivation can jump from related technologies with relative ease. I do agree with your sentiment that it might be easier to sell a jump from php than to, but that is a different problem. If your project is interesting in it self, I'm sure you can find good developers who don't fret too much over what particular language they write in.