It's no contest.
I'm not going to spend any kind of substantial amount of time talking about this, since I'm still guessing/hoping that Boras was the one to initiate conversation in an attempt to drum up some interest in an uninteresting client, but given a hypothetical choice between Griffey and GA, you have to go with Griffey.
He's been the better hitter for each of the past five seasons, every half-decent forecasting system projects Griffey to have a more productive 2009, and while GA is the better defensive outfielder by a significant margin, it seems the Mariners are more interested in landing a DH who only sees the field a few times a month, mitigating that concern. Yes, I'd rather have Anderson as an everyday LF. But as far as I can tell, that isn't what we're looking for.
Griffey comes with his risks, but he's a better bet to be a productive hitter next year than Garret Anderson, and as long as the former is available for chump change, I can't imagine there's any way that Zduriencik would try to sell us on the latter, or that there's any way the city would let him. All things being equal, you side with Griffey. And all things aren't even equal. Griffey's both the better draw and the better bat.