Brooks saddles become comfortable by sagging and shaping themselves to the rider, just like leather shoes do over time. I used to have a leather saddle (maybe even a Brooks) on an old roadster as a kid and it was shaped like a tractor seat, very scooped and collapsed to the point where you could feel the back rail. I bought a Swift a few years ago expecting it to be a thing of beauty but I was so horrified at the crudeness and the weight that I sold it straight on. Oh I also tried it and found it grossly uncomfortable but admit I didn't use it for long enough for it to shape itself to me.
Plastic saddles weigh very considerably less, cost very considerably less, can withstand wet weather, look more elegant and are usually comfortable from day 1, assuming the shape suits the rider's bum. Also they don't sag. For me there's no contest but I'm the sort of rider who seldom exceeds 3-4 hours on the bike and appreciates light weight; it might be different if I was a regular long-distance tourer.