If it's possible to use both approaches without cluttering your API, that may be the best solution. I know a lot of more junior devs that have a difficult time wrapping their head around expressions in C#. Excellent library!
After looking at it, I am concerned it is not possible for a clean experience. We have to give guidance and samples and we have to choose a way of referencing methods in those. Having ambiguous approaches is a bit rough. We may have to just move to expressions.
I completely understand. Either way, I still think the library is great and appreciate the work done to create it.