Depends on the pilot.
In most cases, the P47 would be the one to worry about- people who fly them light are generally out for blood, and they know the machine.
The P51 carries it's legacy in it's back pocket ("P-51 won the war, it should pwn!"). It's also the choice for run-stangers ("Run away! He's got GUNS!"), BUT there's also the few who fly it because it really is an excellent fighter and well suited to actually, you know, fighting. Much bigger population base, so a much more varied threat, from none at all to your worst nightmare.
The bigger threat based on a/c performance alone is the P-51, hands down.
The bigger threat based on what you'll actually run into in game is the P-47- again, hands down.