The firepower of Megador surpasses 5 Imperial II's, but honestly, Executor's firepower should be even greater. On the other hand, Megador should be more durable and better shielded logically, given it has only 3 turbolasers and lots of space for more.
Then you have Megador being probably way, way faster, more maneuverable, given it has like 5 times more enginepower.
Megador 8-9/10.