Just sharing some build wisdom I figured out today after.... well a very long time of not paying attention.
Bottom line: If you don't already plan on taking 4 combat style feats, and you are thinking about taking toughness, take a combat style instead. They always give you more hit points until you have 4 of them (the maximum effect). This is true at every level of experience. It is also true of Epic Toughness except for a hero with d6 primarily d6 hit dice. Legendary Toughness is still superior for HP gain.
Of course, you need to have a fighting style available to you, and a few builds won't unless you put some points in balance to qualify for Single Weapon Fighting (which anyone can do, but you may not want to dedicate the points there)
And of course, it means you have the other benefits of the combat style, if you choose to use it, but even if you don't, its still more hit points.
If you are already taking fighting styles, then it's not a comparison and Toughness is just the only other regular feat to grab hit points with.
Things I learned while making sure this is correct.
The HP you get through Heroic Durability from taking combat styles is actually tallied in the "Enchantments" HP value and not in the "Feats" value. (why is a mystery) Enchantments also includes item buffs and ship buffs.
The HP additions from Enhancements and Destinies are tallied under "Feats" which is likewise odd.
The HP additions from the fighting style feats seems to be gated somehow. So a level 11 druid with 3 Natural Fighting is getting +50% core HP, while a level 19 druid with 3 Natural Fighting feats is getting the expected 75%. It is unclear what statistic the gating is using, be it Level or BAB.