3 blades sound much better!
But - as mentioned, they're heavier, more expensive to overhaul. The weight and larger spinning mass would be a concern to me, personally.
As for harmonics as related to that specific prop and engine combo, that's way beyond trusting an answer on an internet discussion board. Get the TCDS for the prop and examine. Then call Hartzell and discuss. They are, or used to be, very helpful discussing this issue.