"On the whole, citizens are more successful gun users than are the police. When police shoot, they are 5.5 times more likely to hit an innocent person than are civilian shooters." -- CATO Institute Policy Analysis No. 109, TRUST THE PEOPLE: THE CASE AGAINST GUN CONTROL, by David Kopel
While that is a good argument for arming citizens in general, it really doesn't apply here. A policeman is far more likely than an ordinary citizen to encounter an apparent crime in progress. In such a situation, it may not always be clear who the 'bad guy' is (perhaps the guy holding a gun on someone else is an undercover cop, a citizen with a CCW, or a citizen who just disarmed his assailant). As a result, a policeman may sometimes misinterpret what's going on and consequently shoot the wrong person. By contrast, a law-abiding citizen who uses his gun for self-defense isn't going to have this problem. He may be just as likely to encounter a crime, but he's more likely to be there at the beginning. An armed citizen who's attacked by a mugger doesn't have to guess who the bad guy is, and consequently isn't too likely to be wrong.
In an airplane, it will be pretty obvious to everyone who at least some of the hijackers are (there may be sleepers who will not be obvious). Law-enforcement personnel will not be at their usual disadvantage in such a situation. Consequently, I would expect them to do no worse than ordinary citizens at identifying the right target.
There is, however, a different disadvantage to law-enforcement personnel: unless there are numerous armed personnel it would be a simple matter for two terrorists, seated on opposite sides of the aisle one row apart, to jump and disarm any LEO or armed guard that walked by. By contrast, if there are 15 secretly-armed citizens, a hijacker who disarms one will still be outnumbered fourteen-fold.