What is more important?
Note: I do mean this in the black and white manner that I've just put forward, there is an explaination below.
A few thoughts on the matter
I've had this conversation with a few people before, a lot of people seem to think that it depends on quite what the situation is, a mass murder should not go free just because an innocent man may be imprisoned. I've heard the "think of the people that the murderer may go on to kill" argument, however try this on for size, how would you feel if you were wrongly imprisoned (or in some contries killed), you'd done nothing wrong but you have in fact been "stitched up" for real. If the innocent is punished, there is no gain, and the real murderer may go on to kill more.
Then of course comes the "well what's the ratio required?", what if the 1 innocent was you?
If the innocent are punished then an entire society is guilty of being an accomplice to the punishment. Does the sentance get watered down for 100 people watching a murder just because 100 people watched it rather than 1?