The reason he wants open testimony is so that at any time, he can claim that information is "classified" and can't respond. In a closed session, the only congress people in the room would be people who have the clearance to hear classified information, and would have oversight authority over the FBI. He would have no place to hide in a closed session.
Mueller might just step in and say....Oh no....you can't testify to anyone Mr. Comey and you will testify under my jurisdiction only.
I thought it might be something like that, but I don’t know enough to say so