A Ruger P89 will compete in USPSA Production and IDPA Stock Service Pistol.
You need a holster, magazines, magazine carriers, eye and ear protection, etc. For IDPA you need only 3 magazines but will wear a concealment garment (vest, jacket, big shirt) and must have the ability to draw from under it without getting tangled up and endangering yourself or others.
For USPSA (IPSC) you will open carry but will need more magazines because the stages are longer.
Which is "best" depends on what is being done in your area. If both, check them both out and shoot whichever you like or whereever the people are nicer.