Yeah Ash, but Nirvana (as ace as they are/were) were Grunge, and not necessaraly 'Heavy metal Rock'.
They're in a different class technically, but both at the top of each.
Nirvana are still VERY good even though they're no longer together (alive or not) you can still listen to their tracks, and they still beat any grunge band that are around now....and will for a long time to come.
I feel this will be the same with Metallica when/if they ever break up.
AD/DC have been going longer admittedly, but when it comes to it, Metallica kick their ass. I feel that AC/DC have had their time, and they were good when it mattered. But overall I think Metallica are better.
But saying that, you can't beat a good bit of 'back in black' or 'thunderstruck'!!