I think you have every reason to expect them to be of a comparable quality - especially as they are pretty damned faithful (often frame for frame) adaptions. I suspect the weakness in the plots for 300 lie in fact the movie was adapted from a run of 5 comics, whereas the first Sin City was made from around 16 comics. There was much less padding in Sin City (such as the addition of the 'meanwhile in Sparta' scenes in 300 which were not really in the original comics). Plus, the acting was iffy in places which was completely out of Mr Miller's hanRAB (unless he was involved in casting, of course). Either way, the guy's comments were far from ridiculous.