Sin city has film noir cool dripping off it, but it deteriorates fast until the point where Frank Miller makes them either an unintentional spoof of the series or just plain unreadable.
As for The Invisibles, well it starts off slow, towards the end you won't have much of a clue what is going on until you read it a few times, but its by far the greater series overall. For this reason, I would choose The Invisibles.
Grant Morrison may be incomprehensible to some, but he still delivers some of the best stories out there and delivers more 'bang for your buck' as they say. Of the big name writers, he is the closest we will get to a visionary these days. Miller could only ever really do brooding,style and a bit of the old ultra violence. Today he is left nursing a decaying legacy, whilst Morrison is still blazing away.
Still, Miller delivered the immortal line 'I'm the goddamn Batman!'
Perhaps he still has something left.