Re: metalmark
The masterworks volumes are essentially collections in chronological publication order, irrespective of story arc. The standalone sagas like Dark Phoenix, Days of Future past, Brood War saga are collections of issues that cover that story. Some of those have continuous issues, others have gaps between published issues.
If you just want great Xmen stories, go for standalone story arcs. If you really want, you can go Xmen completist for the Claremont era (issues 94-280) as he is the master of foreboding and the long game in his storytelling. It becomes even more of a commitment following the publication of New Mutants, and you kinda need to read both to get what's going on. Actually, to be fair, New Mutants was the stronger book for a fair while there, and could be read on its own, but Uncanny Xmen relies on some big story beats from NM to make sense, particularly around the introduction of key characters.