Actually, I would make the case Heisey is not below average in CF. He had one bad game recently and people remember that. Yet he has been above average when playing CF IMO. Prior to this year I would say Stubbs was superior, but like I mentioned, he has not been to that same level this year.
I can only guess Stubbs is dealing with some mental issues, both at the plate and in the field. Now this is only guessing, but he seems to be the kind of guy that beats him self up mentally. He needs to go to the Brandon Phillips school of playing mentality. Phillips can make the biggest bonehead play and then turn around and make the defensive play of the year. Things just don't get to him. On the other hand Stubbs seems to be affected for a long time by his mistakes and it just keeps on building up.