Aesthetically, I have a slight preference for the 587, so that's my vote. It's my favorite modern Panerai.
But I will always remember Sergio comparing the two cases and noting a slight comfort advantage for the 673. So this brings the two quite close in my mind.
I am doing wrist strengthening and enlarging exercises, so I can wear both, simultaneously or one at a time. Regardless of how much time they spend on my wrist (or not), they're both beautiful works of horologic art.