6 hipoints is not worth more than one feat. Toughness is crap, we all know that. If toughness gave 6 hitpoints, it would still be crap. Unless toughness was set to a ridiculously high number, it'll always be crap if it remains static.
Now as for all this feat vs. spellcasting level bull. You cannot directly compare the two. Some feats are more powerful than others, and some spellcasting levels are more powerful than others. 1st and 2nd level wizard spellcasting levels are certainly not worth 4-5 feats, otherwise every fighter known to man would take two (or more!) levels of wizard. Most don't.
However, 17th and 18th wizard spellcasting levels are probably worth 4 or 5 feats, but it's irrelevant, because you can't just do that.
You're comparing apples and oranges. I'd never allow a feat that actually gave one or more spellcasting levels, regardless of prerequisites. +1 to effective caster level? Sure, but you certainly would never be able to take it more than once, and your caster level would have to be lower than your character level.
People keep saying EK is overpowered but it's not. You give up two spellcasting levels to get it, and if that's all you give up, you are going to a mere +15 BAB at 20th level. Congratulations, your BAB is the same as a cleric's, except clerics *don't* lose two caster levels for it and *don't* give up class features for it, and *can* wear armor.
So why is EK broken again?
-The Souljourner