This is a good question and a tough one for the coaching staff to make.
On one hand, competition does bring out the best in players, and if Pennington really is better than Henne going into next season, then he should have no real problem beating him out.
Though Pennington has proven this year that he is not only a quality starter in the NFL, but a top 6 or 7 QB in the game. He may not light things up with a lot of touchdowns, but he is smart, knows the game in and out and makes few mistakes. We will be very good next season if we don't have a QB switch.
Henne still remains the QB of the future, and while I am sure he wants to start now, I don't think the coaching staff would mind sitting him for 2 or 3 years(like Aaron Rodgers) as long as Pennington is playing at a high level.
At least if their is a QB competition next year, it will be between two good quarterbacks. Unlike previous battles(Feeley, Fiedler) that were just to figure out who was less worse.