Go for the Hayes :-$ !!
The Hayes Comps are actually better than the nines that they have out now. Although the Avids are good brakes, the Hayes will give you better modulation and increased stopping power. After switching from mechanical to hydraulic, I can't see myself ever going back. Unless I become a full on XC rider, which I don't see happenning.
No offense to any XCers, as I still ride XC once a week or so.