Here's a quick look at the top 5 SP that were available at the time and what they have done since signing their respective deals.
Barry Zito- 7/126M, 40-57, 4.47 ERA 767.3 IP
Gil Meche- 5/55M, 29-39, 4.60 ERA 616.3 IP
Ted Lilly- 4/40M, 54-38, 3.66 ERA 781 IP
Jeff Suppan- 4/42M 32-42, 4.98 ERA 645 IP
Dice-K- 6/52M(you can debate the posting fee all you want, but as it has no bearing on his AAV, which in turn has no effect on the Luxury Tax. So it does not effect the Sox operating budget like it would if they signed him to a 6 year 102M contract)- 46-27, 4.18 ERA 585 IP
He is not as over paid as Zito. Suppan is a RP now. Meche is struggling to stay healthy. Lilly has been the best of the group. But he's been pitching in a fairly weak NL Central for the better part of his contract. The Red Sox got at the time the 2nd best SP out of the top 5 for the fifth highest AAV amongst the group. And out of the 5, Dice-K probably has the best upside over the next 2 seasons. I would say Lilly. But I'm holding his age against him in this particular comparison