the deal for damage is.
heroic strike transforms one of your normal attacks into a special attack witch deals additional damage, but this attack wont generate rage, but use it instead.
slam is a additional attack of your weapons damage, this attack resets your swing timer, so its essential that you use it just after you complete your normal attack.
example
with a slow 2h weapon dealing deals 70 damage every 4 secs. you can with improved slam spend 0,5 sec to deal another 70 damage. if you use heroic strike you will spend 0 secs to do another (i can remember how much exacly at your lvl but about 30 aint it..) BUT you also set your rage generation to 0 for a whole 4 secs (exept for what you get from getting hit which aint alot) so in this case slam would be prefeerable.
dual wield: 1h wep deals 25 dam every 2 secs, and another weapon dealing 10 dam every 2 secs.
here a slam would spend 0,5 secs to do 25 damage.
and heroic strike would spend 0 secs to do those "30" damage.
heroic strike also sets your chance to hit at a higher rate if you are dual wielding since all special attacks have a set chance to hit.
Buzak