In a historical sense, this really isn't an argument. The Ravens' 2000 defense was far better. They were mentally tougher, and perhaps more than any other defense in modern NFL history, they carried their team to a Super Bowl victory.
But physically? In terms of sheer talent and potential? I think the Ravens' 2008 team can be better. It's like arguing that Paul Thomas Anderson is a better director than Martin Scorsese. No, Anderson doesn't have Scorsese's overall body of work. But there is definitely potential there.
For starters, the 2000 defense didn't have Ed Reed, who might just be the best free safety in NFL history before it's all over.
Ray Lewis might not be as fast or as strong as he was in 2000, but he's still one of the most feared players in the league, and he's probably even smarter about recognizing what's about to unfold. Terrell Suggs and Bart Scott are also more versatile and better athletes than Peter Boulware and Jamie Sharper.
The one area in which this team simply can't come close to measuring up is at cornerback, because Chris McAlister and Duane Starks were lockdown cover guys in their prime. But it was also easier to cover receivers back then, before the NFL really started enforcing the rules regarding downfield contact. Fabian Washington and Samari Rolle don't have that luxury.
Ultimately, the 2008 team has to prove it in the playoffs to even make this a credible argument. But right now, it's building a pretty good case.