Depends which united card you have. Eg United gateway no AF card doesn't include priority boarding, United Explorer does.
Economy plus is just extra legroom (3-4inches or bulkhead ~more~ legroom).
Are you sure you're not asking about premium economy? Generally economy plus costs 29-299 more per leg(long haul would be 150-299). united also offers buyup to premium economy, as low as $300 . Premium economy is different (wider/recline/legroom/features) seat with different meal