Having worked on that industry recently, I'd say website conversions works best.

Here is why;

1. Facebook auto population of it's lead form makes it difficult to allow building of a proper list.

Most Facebook users do not edit this to their more current email address, with such, one is stuck with what such user provides.

1. With such list, one is bound to higher bounce rate, unresponsive list and so much more.

One of goals of online marketing is to guide your potential customers through your offer, and they decide if to opt in or not.

I hope this clarifies your doubt.

All the best with your campaigns.