Short summary: go with the dex!
Firstly, HDEX >> AC any day of the week even if all you ever care about is defense. The reason being that heroic dex adds to avoidance. I forget precisely which of the two it is and I can't check my notes from here, but I think it's parry and ripost from dex and dodge from agi (block too, but we don't get that). So in pure damage mitigation terms a lot of Hdex makes a big difference. Of course it may not be possible to parry a rampage hit as it's not directed solely at you - I'm not sure infact on that one.
Secondly, HDEX adds to +damage in much the same way that HSTR does, but at a different part of the equation. See http://www.thesafehouse.org/forums/s...ad.php?t=37115
for more data.
Specifically an attack is damage_bonus + str_mod + modifiers * (dex_mod + hit), where hit is a combination of DI internal (1 to 20 random), weapon damage and damage table (1 to table_max in .01 increments). Note specifically the str_mod (hstr/10) and dex_mod (hdex/10) are in different parts. Str appears at the end and always works. dex arrives near the start and can be multiplied up by modifiers - either crit, rogue's fury, whatever. However some modifiers can work against it such as the minimum hit modifier in worn fero; dex gets applied, then it's checked against min hit, and finally increased if necessary possibly making the dex component irrelevant. Note that dex (normal, not heroic, but heroic grants an overcap normal too) has a very small boost to crit rates.
In pure +dam terms though dex >> str in every way. To offset against this is the fact that str boosts attack which in turn skews your DI hit distribution away from 1 and towards 20. Whether or not more attack really helps much is questionable. Probably we should do some real world parsing with and without fero to test it. My last tests were years ago on attack and crit rates. I have enough heroics now to test again.