So the choice is
a) +2 Con for
- +1 HP every level (retrospective back to level 1 (total of +20 HP by level 20))
- +1 to all Con Saves,
- +1 to any DC based on Con,
- +1 to any other Con based roll (such as using HD to recover HP)
-OR-
b) "Tough and Stringy" for
- +1 HP only when rolling HP for a new even-numbered level, (ie 5 HP, 4HP alternating)(If taken at level 4, that's +9 HP by level 20)
- and no other benefit!
I can only think of one marginal case where it's a benefit, and that's a Goblin who already has Con 20 (9HP/level) and can't increase Con further. But surely such a Goblin Hero has better uses for a Feat?