Like has been said, a lot depends on what you're going to be doing. There are a lot of variations of what is commonly called "trail riding". For me, trail riding means a loooong ride in the mountains. That means I have to carry food, maybe for multiple days, flashlight, gloves, maybe a small first-aid kit, and any number of other odds and ends. While I love leather, leather saddle bags of any decent quality will set you back anywhere from $80-$150. I like canvas bags. They last forever and are very water resistant. I like a large set, in the 10X12" range. I don't do any cantering on the trial, so I don't worry about flopping against the horse's kidneys. I have had occasion, though, when a faster gait was necessary and the horse didn't seem to mind that much, for a short burst, but it is rather ungainly for a long canter.
My father tried one of those full-outfit cantle bags that has saddlebags and a large zippered bag for a sleeping bag or coat, etc. He really didn't like it much. Wasn't as versatile as our large canvas bags. Also, the canvas seems to be more durable than the nylon. Just my experience.
The canvas saddlebags in the photo are about 40 years old, and have suffered through hundreds, if not thousands, of trail miles. I have to admit, we don't take good care of them either. Tough bags.