In general, I think it's easier to run off of a runsheet, but it depends on the show. A runsheet/runbook/WWW is more portable than hauling your script around, and if you need to cue something off a line, you can always include that cue line in your paperwork. As an ASM, I find that I spend most of tech making updates and changes to the runsheet anyway.
However, with an opera, since the ASMs generally cue every entrance, it's easier (in my opinion) to run out of your score. (If the score is too cumbersome you can always get it bound at a copy place.)