My problem with this type of thinking is that is too easy for Y to swoop in and take over X for you. They have the team, reputation, and recent success. You, most likely, have nothing.

Examples: "We are facebook for bicycles!": that's just a facebook group. "We are Heroku for PHP/Clojure/Python":http://blog.heroku.com/archives/2011/8/3/polyglot_platform/

These are my best examples, but if X is just a feature that Y could implement or acquire, then making that comparison could be a major mistake. If they expand into your niche, then your X of Y idea backfires on you hardcore.

Edit: I saw that my variables were reversed at some places.