Katana is actually better. Cos you move REALLY fast, making you able to cut one clots head while running trough 6+ clots without taking damage. With chainsaw, it is impossible. You need to stop and fight.
And I dont see using katana to hold a doorway more difficult than chainsaw. I can back off and switch to fire axe anytime I need to, and again, without taking damage. Chainsaw just dont kill trash fast enough.
An most importanly, katana can stun gorefasts with primary fire. In narrow hallways, you can stun those two gorefast leading the swarm (instead of aim for the head to kill them) and back off to do anything you need and then kill them while chainsaw take too much time to do two alt-fire dont give you speed boost.