Asomugha is a true shutdown corner!? And you figured this out how? I ask, because every time I watched the Raiders, you'd see the teams lined up, Asomugha up at the top of the screen, and then after the snap, the camera follows the QB, and Nnamdi dissapears offscreen the other way. You could argue that the evidence is that teams didn't throw his way, but that really just proves that he was covering better than the other guys in Raiders' 2ndary. It's not like the Raiders' pass D was really above average.
Furthermore, as we've seen with Deion Sanders, Champ Bailey, and even with Asante last season, reputation can be a big influence on whether QB's will throw someone's way. The one time I saw a team really challenge Asomugha was their game against our Pats, and Cassel was completing passes on him all day.
So, no, I don't think you can say that Asomugha is any more of a "sure thing" than Peppers until you've seen him play on a defense where it isn't so easy to just avoid him. If anything, I think Peppers may be more of a sure bet -- we've seen him be a pass-rushing terror, and that's entirely system-independent. He's also a much rarer physical specimen than Asomugha -- you just don't see guys with his speed and agility at that size. ((This is why BB devoted so many early draft picks to our d-line.) This is another advantage that isn't system or situation specific.
Anyway, the whole conversation is rendered moot by the fact that there's no way the Raiders are trading anything for Cassel. Like him or not, they've got to much invested in JaMarcus Russel to go trading for another QB.