I recommend reading here: Western Saddle Fitting and Different Tree Sizes
The big problem is that one manufacturer's full quarter horse bars may be significantly different than another - so unless you have checked that brand of saddle, you don't know.
Trees used to be made from pine. Pine isn't the hardest or strongest, but it has some flex to it, making it more comfortable of the horse and more adaptable. Ralide is strong, but it doesn't flex. Many folks are now making 'flex tree' saddles, which are an attempt to make a plastic tree with the flexibility of pine. The plastics are lighter, don't warp in rain and easier to make.
I had an Abetta. It was a very good value, but it didn't fit my horses. It had full quarter horse bars. The width was OK, and the curve along the back was OK, but the tree was too long for their short Arabian backs. I ended up giving it away to a friend of my wife, and I gather she loves it. I seriously considered buying an Abetta with an Arabian tree, but ended up buying an Australian style saddle instead.
If you want enough info to REALLY make your head spin, read:
If it is like its english counterpart, it will tend to make you think every horse needs a custom saddle, but most do not. But it has lots of good info on saddle fit and what to look for. The tough part is that you just don't know until you put it on the horse's back. If you buy online - and all of my saddles were bought online - make sure you can return it. It will run $35-100 to return a saddle, but that beats trying to sell it at a loss, or putting up with a saddle that hurts your horse.