When freelancing / contracting a customer will typically specify functional requirements, acceptance criteria, etc, and the implementation details are in the developer's hands.

As a developer your technology choice is a balancing act between what you are most familiar with, technologically what the right tool appears to be, ease of finding coders with this skill and their expense, and a few other factors.

I'm in a situation where I have evaluated my options and selected a somewhat obscure open source technology that I believe will get me there faster, easier, and be more maintanable in the long term. It's different, but I think that that is what the requirements call for.

The customer has inquired about what I'm going to use to build the solution, and now they are concerned because they've never heard of it before. The reasons for my choice are mostly technical, whereas the customer isn't (but they know some buzzwords!). Explaining these technical reasons will not be easy, and I am not sure if that is the right way to approach this situation anyway.

And that's my question: what is the right way to approach this situation so as to cause the least amount of headache for everyone involved?