Of course Colon is more valuable than Willis. He is a consistent proven ace and workhorse. Willis is a rookie that is going to go through rough periods and will get lit up from time to time, especially as teams see him more than once and scouting reports get out. Consider his last two starts against the Mets. His first start, he dominated, 1 hit in a complete game. He faces them again the next start and gave up 9 hits in 7 innings. This indicates that he is more hittable second time through and if he was playing a team that actually could hit in the clutch or capitalize on its baserunners, he could have given up way more than 2 earned runs.
If you think Willis is going to keep his ERA in the low 2.00's, you are dreaming. At best, it will go up to around 3.50 as the league figures out how to hit him. Also, don't expect the wins to keep up either; he has gotten lucky on run support so far and the Marlins are a weak offensive team that will only get weaker once they start the inevitable trading off of Lowell, etc. Colon is on a team whose offense seems to be coming around and there are also rumours he could be traded to a contender.