Use what works best for you. If a seat is too soft, though, your "sit bones" can sink into its padding far enough that now your surrounding tissues (muscles and Other Things) start bearing weight too.

What I like is a saddle more along the lines of a racing saddle, like WTB SST models, with firm thin padding on a flexible base that has some sag to it. I own a Flite Carbon that takes this to the extreme... zero padding, just a very springy carbon shell. And if I'm in shape, cranking out a long ride on it isn't a big deal (given a bike that isn't a sledgehammer to ride, anyway). If I'm not in shape, then anything will hurt after a few hours