I may be guessing here, but I suspect the OP is not a professional, at least not yet. Otherwise, why ask? I think the answer comes down to either economics and/or ecosystem. FCPX is pay once, and then essentially never again. PP is subscription - stop paying means stop using. By ecosystem, I mean who can help you learn this, and what your friends/cohorts/clients are using. They are both very capable programs, though FCP doesn't necessarily show everything at first look. One of the areas FCPX shines is ENG - it's very capable and fast with edits that enable you to put together a professional looking clip in short order. Easy, too, once you know how!