Looking at the short term impact, Hamels should have more value. Liriano's outings will be slowly lengthened to work his arm back into starting shape. Like has been said on these boards before in one of the countless Hamels threads, pitchers usually have more success the first time they face hitters. (Ex. Felix, Duke, etc.) Therefore Hamels should be the better option for a couple months or so. However, I think Liriano will be the better pitcher in the long run. He does not have the injury history of Hamels and he projects to be similar to Santana.
PS. I'm quite mad at Hamels for his injuries. I dropped him off my minor league roster at the start of the season for Lastings Milledge b/c he couldn't stay healthy. Of course, Lastings will help my team down the road, I could just use another arm right now.