Hasbro's Hardhead head is more G1 toy accurate (and remember that the toys came first), whereas TakaraTOMY's is more media-accurate. So both are G1 accurate, it just depends on which G1 source you're looking at. Both Generations and Legends Brainstorm feature G1 toy and US cartoon accurate heads (inspired by IDW) rather than being G1 anime-accurate.

I don't get Hasbro's Skullcruncher head though.