I think if you want our purchase this round to be a step on the road of making our library great then back-fill is the way to go. The Library of Mournings is cool and impressive and having copies of its books in our library is a good long term goal, but right now it is a very inefficent way of using the library's resources. Compare those two options:
Assuming that 'all available books' goes up to Esoteric, the second option means getting 500gc worth of books. On the other hand, getting books from two different cultures on three different topics is 1500gc worth of books, three times as much. We don't know how much we'll get from picking back-fill (and we never will if we keep getting Eonir books) but it will be even more than that.
I can see the reasoning for why having engineering and linguistics books will be good for the library, but pick almost any topic in our library and you could probably make a compelling argument for why more books on that topic will be good for the library. We are probably going to want to start negotiating with other libraries soon, and while having two more shelves of Eonir books isn't bad, quantity has a quality of its own.
And it's not like we always know in advance what knowledge we'll need. Having lots of books on lots of topics isn't some noble sacrifice for the sake of the library, it's a good long term investment for Mathilde as well.