This is my own analysis...the data was taken from here.
-David Harris has more sacks and ints over the last three years, and is more durable than Willis.
-Harris has 70 less snaps than Willis (played on better teams), and accounted for 65 less tackles than Willis over three years. However, Harris still has a better stop rate percentage, and his stops were closer to the line of scrimmage than PWillis' (indicating that Willis' tackle numbers may be inflated due to him stepping out of his responsibility and making plays down the field after his teammates were unable to).
-Over the last three years, Willis was responsible for an average of 17.6% of the plays he made on defense for his team. Contrast that to Harris who played in less games and had better personnel around him, leading to an impact of 14.6% of the team's plays over three years.
-Last year, when Willis' defense finally played up to the level the Jets have been playing at for three years, Willis' tackles production in the time he was actually on the field dropped off a dramatic 22.5% (after accounting for the time he missed due to injuries).
Just by going with the eye test, I'd go with Willis over Hitman, but it is definitely as close as I thought it was, and not the joke the others on here have made it out to be.