Keep the systems separate
Lexile and ATOS are different systems. Displaying them together can be useful, but their values should not be treated as interchangeable.
Begin with the label attached to the measure and avoid inventing a conversion when the source does not provide one.
Notice the kind of book
Prose, verse, and graphic nonfiction place different demands on a reader. A nonnumeric code or a missing measure can still be useful information; it is not a verdict on quality or fit.
Use measures to narrow, then preview
Filter to a manageable group, then read the description and preview a few pages. The final choice can account for both reading comfort and genuine curiosity.