If you must just choose one I would say Pitstop is the way to go but, keep in mind that Acrobat Pro does have some pretty good preflighting and fix capabilities and, pdfToolbox is also a very good product.
A copy of FlightCheck is well worth the money. Always has been, always will be. I've spent too many years playing with all of the tools and no one does it all perfectly. Think of it this way, it's like securing a building. If you really want to "filter" what comes in you have to approach security in a layered manner. Same thing with firewalls and virus protection. "Ogre's are like onions, they have layers..."