Well, you should. The first ceases to be about the band (it's never really about the band, but is based around them as an entity earlier on) about half way in and becomes a fantastic study of the characters of The Flaming Lips themselves. the entire story centering on Steven Drozd is just heartbreakingly honest and painful to see almost. The film is filled with shifts from the heartbreaking to heartwarming and it's both compelling and involving and I loved it. I'm scared to watch it again because I don't want to weaken it's effect.
On the contrary, I could watch Sin City all day, it's stylish, brilliantly shot, echoes the source material perfectly (so it's violent, striking, interesting, the characters are detailed and involving) and the film looks just like the comic did, with almost every frame from the 3 volumes of the comic book in the movie, actually in the movie. Great fun, well performed, and it's always a joy to see Mickey Rourke.
I can smell the Pulp Fiction comparisons, but I think I actually prefer this..
Warning: Opinions may be coloured by the fact it's 1:15 am and i've not slept since Saturday morning.