In my non-expert opinion, I think spread formations are more productive and efficient in general. I think Chan showed us that. You spread the defense out, it's easier to find soft spots for WR/TEs, and holes for RBs. I honestly don't understand why any team would run anything but a spread offense. I mean, for short yardage plays, I can understand a heavy formation, but that's about it.
Edit: Now that I say that, I can see a spread formation being difficult for a QB who is slow to scan the field, and recognize coverages/openings... hm