I think any fan would love to have both of those talanted guys on their favorite team's squad, but I actually dont think we need either one to go 16-0 (hypothetically speaking, of course).
Of course, if the question is more focused on which player could have a greater impact on a team, I would say Brian Urlacher. Though Bailey is very talanted, I have always believed that the front seven (and the LBs in particular) are the heart of any defense and that a solid front seven can make a mediocre DB field look good. Therefore, I'd say a top-tier LB like Urlacher would simply have a greater opportunity to improve a defense as a whole than a CB like Bailey (though I am definately not underestimating the importance of good CBs).