If the defender spends a conservative 2 surges per combat, then they get...
Swift Recovery: 6 / 8 / 10
Disciple of Stone: 10 / 10 / 10
Superior Armor: 5 / 9 / 13
Toughness: 7.5 / 15 / 22.5
Swift Recovery leaps up further if you consider after-battle healing.
You're not calculating total maximum hitpoints for the purposes of the evaluation.
The defender's role is to mitigate attacks, and make the undoing of damage more efficient. The extra five maximum hitpoints from Toughness have not mitigated ANY damage. They have not made the use of healing surges more efficient. The only part that does is the increased surge value.
The only time the extra hps from toughness have value in three specific situations:
A- You got knocked down to 1-5 hit points (1-10 paragon, 1-15 epic) exactly, and thus having toughness prevented you from dropping, saving an action (an admitted tangential bonus) The irony of this bonus is that it's only a bonus if you can say you were still standing on your turn. If you still get knocked down before you can take the action you won... then Toughness may have actually cost you hit points because the healing surge you spend to get back up will be short the 5/10/15 extra points that knocked you below 0. Thus your surge spending becomes less efficient.
B- You got knocked to 1-2.5/tier hps above the bloodied line against a monster that deals extra damage to bloodied enemies
C- You got knocked below 0 hps, and dying to being at nevative bloodied actually matters more than death saves.
These are the only times Toughness matters. If you're only spending a conservative 2 healing surges, you only went to bloodied, and A and C are not relevant to the situation. B is very situational. And you know what? B is a situation that could be avoided by say... 3 damage not happening at all. If 2.5 hit points are the difference between bloodied and not... then not taking 3 hit points at all means you never got bloodied. SupArmor is superior in situation B.
In actuality (assuming heroic), Swift Recovery would have erased 6 points of damage, DoS mitigates 10 (provided you don't have other thps at the time you spend those surges), Superior armor mitigated 5, 7, or 9 depending on item quality. (You start getting +3 items at 7th level), and Toughness erased only 2 or 4 damage, depending on rounding of numbers.
To put that in perspective, at second level, a Battlemind with 18 Constitution will have 37 hit points. Assuming +1 armor is fairly fair for this guy.
His surge value is 9.
DoS gave him an entire free surge. Not just free as in he didn't have to spend a surge... but free as in those two actions that were used to allow him to spend surges ALSO gave him a free extra surge
at no action or resource cost whatsoever outside of taking the feat.
Swift Recovery gave him 2/3s of a free surge.
Superior Armor gives him over half a free surge.
Toughness gave him two free hit points. Don't go spending those all in one place!
For toughness to start favoring comparably to Superior Armor... you have to be spending four surges that combat. And you know what? Then toughness's other benefits come into play, cause that's all your hitpoints....
...on the other hand, you probably wouldn't be close to death at all with swift recovery or disciple of stone, would you?
Toughness is always better than the superior armor, especially since the superior armor also has an additional -2 check penalty over scale or -1 AC penalty over plate. That said, it may well make a viable second feat.
Except the extra maximum hps from toughness do not translate into additional resources on the same level as the armor. You're looking at feats which are literally granting you entire free healing surges that you're not paying surges OR actions for... and you want to talk about Toughness which does less than any other feat at giving you free healing?
Ringmail is more viable because there are characters who would want a light version of chainmail anyways. I'd never take the heavy version... but I also tend to take more proactive feats than the ones on the list.
Most classes than can use Chain Mail can use Hide Armor... if they wanted to use light armor, they already WERE using light armor.
You do have a point at the higher check penalty, and vs plate for scale classes, I do agree. The first is a playstyle thing tho. The latter is a real question and isn't as easy to evaluate as surge-boosting does for defense.
It depends a lot on how your campaign runs and it's quite situational. All I could suggest is take a note of how many attacks you take, and how many hits you take, how much damage you take, and how many surges you spend to erase that damage, and how many combats that means.
It's statistical stuff, most gamers shouldn't bother, and it's way beyond the scope of this thread.
But... it'd be interesting.
Also, for a Warden, there IS no upgrade like plate from scale... hide is as good as they get, and there's no trade off between a higher tier armor and taking the tough armor.