How seriously are you planning to do endurance? Since you're looking to buy a horse that would be able to do endurance, I would recommend putting yourself in the best possible scenario for an endurance horse. Mustangs can be successful in endurance, but you never know. The bay mare seems taller and leaner, which is a major plus in endurance. However, either one of these might be terrible since you don't know how they move, metabolics, or desire to go over long distances. Not every horse is cut out to be an endurance horse, and you'd be taking a big risk if you were expecting any of these to be good at endurance.
In other words, you need to decide what's most important to you. If you want to be a serious endurance competitor, I would recommend looking at an older, broke horse that you can assess the things I listed above, especially the will and ability to travel long distances without wearing out or giving up.
And even though a horse can compete in LD's at 4 years old and endurance at 5 doesn't mean that he she should. You're going to need to break the horse and then get it trail ready and race ready without pushing her body and especially her legs too far too soon.