Rewatching the Haynesworth head stomp, I honestly don't think Garrett should get a harsher penalty. For one, Garrett was getting drug away when Rudolph charged him and he swung his free arm with the helmet. In the heat of the moment I can see where you might try and defend yourself like that while you're being restrained. Secondly, he didn't break skin, unlike Haynesorth's attack that required 30 stitches. Third, Haynesworth's attack was well after the initial whistle and he was not locked up with anyone, he stomps on Gurode's unprotected head in an almost emotionless way.
That said, I thought back then that Haynesworth deserved a harsher punishment than what he got, and I expect Garrett gets a longer suspension than Haynesworth, but if you use that incident as a barometer I think Garrett's swing was clearly the lesser of the two attacks. Rudolph was not defenseless unlike Gurode.