For the Rider

There are people with more knowledge and experience than me in determining how a saddle fits the rider. You'll go a long way paying attention to your own comfort and level of work in your riding. Riding shouldn't be work just because of saddle design. 

If the twist is too wide for your anatomy, you will ache. It's not necessary, find a narrower twist. That's probably harder for western riders than english. I prefer a fairly narrow twist and once sat in a Dave Genedek saddle designed for a woman. Wow, heaven! So it's doable.

If you are having a difficult time keeping your feet back (common demand from riding instructors!) or your thighs ache from posting, your stirrups are probably positioned too far forward. A few (very few) saddles are designed for a woman's pelvis and foot size. Nothing but public demand is going to make this any better.

But here is a rule of thumb that gives you something aim for. Sit in the saddle with your legs dangling out of the stirrups and your back in a neutral (not arched) position. Use your hands to determine where your seat bones are and find a spot on the side of the saddle so you can locate the front-to-back position from the ground. Imagine a line drawn from there straight to the ground. Then look at the line your stirrup leathers make to the center of your stirrup. Measure this either with a ruler or a hand-span. 

For many people, the distance from their heel to the ball of the foot should match the distance from where your seat bones were (deepest part of the seat) to the stirrup hanging. If the stirrup is forward of ideal, you will have to work hard to post and fight landing hard on the cantle.

Posting should be effortless. The horse bumps you up, gravity brings you down. You should be able to do this all day without any muscle strain. 

A few saddles offer variable stirrup positions, Free'n'Easy, Specialized, and Ghost Treeless saddles along with several treeless models. I've modified the stirrup position on a couple cheap western/trail saddles, but in most cases, this isn't possible. Again, this isn't something that will change until most people realize they should stand in the saddle, not sit in it as a recliner. You're more likely to land on your feet when your horse does the 6' arab teleport trick if you're not already sitting!

Good luck in finding the best saddle for both you and your horse. And don't discount your own comfort, ultimately it will affect your horse.