When you get down to it, they both need to fit the horse and rider comfortably. This is something I started learning in the english riding world and now am carrying over into getting back to my western riding roots with Buddy.
I'd like to share some valuable information for other riders out there (and so I can reference it all in one place--here). These Schleese videos on Youtube are great and I highly recommend following Schleese Saddlery, whether you ride english or western. I've learned a LOT from these and from attending saddle fittings with Schleese and other saddle fitters, albeit for dressage saddles. And I've learned a lot from my horses, like how much good saddle fit makes a difference to them. Happy horses mean more pleasant rides and fewer problems, especially in their backs and legs.
I'd wish I'd known all this when I was growing up--but the internet wasn't around back then. I'm glad I know it now so I can buy the best western saddle for Buddy next spring. Then we can do both dressage and western dressage, plus not get laughed at while chasing cows, although my dressage saddle is far better than any western saddle, and I grew up riding western (or maybe because I grew up riding in a western saddle).
After seeing the intro in the first video, I suggest that you skip to the 2:42 mark on each.