How many of his games for Parramatta were as a starter and how many were off the interchange bench? I think you'll find most of them were off the bench as he had his brother and Michael Vella in front of him as the starting front rowers. He knew if he came to the Roosters he would be a starter. As such he may have stayed if they'd gotten that hack Vella out of the starting side. The Eels made the wroing call there eh?
As to the wider issue, whilst Jason was at the Roosters he was a very good front rower, in terms of skill I think JWH is the better of the pair. He is better as a yardage getter, has better lateral movement than Jason did (which was about Jason's only weakness), is a more inspirational personality type, and has far better offloading skills.
