Tougher: Pre Heresy or Post Heresy Marine.
I'd have to say Pre Heresy marines were tougher. Not that the tech has changed a lot in 10K years, but they were the Originals, and had'nt fought Orcs, Elder or any other xeno's before so that was a first. Plus they came directly from the Primarch's geneseed. I know all the geneseeds came from the primarchs, but those were like a new car, with no miles on them. Not that it makes that big a difference, but in my minds eye it just makes sense. Also, being able to fight with their primarch, actually being around them would make them fight all the harder.
Those are just my views.