Ironically, I think that the answer is "both". If you want stunt and action options to add on to regular D&D, that's Book of Iron Might. If you want to go with a campaign where there's little magic and all action, that's Book of Iron Heroes. (Warning: the latter has heavy errata. You'll want to download it if you use Iron Heroes.) I have both and I use both depending on the need. Generally, I run Iron Heroes as a stand alone game.