That's not the 'single effective argument', even though it is a very powerful one and one I agree with.

But there are four cases to consider

* found guilty but actually innocent.
* found innocent but actually guilty.
* found guity and actually guilty.
* found innocent and actually innocent.

Leaving the last two aside: the first is the Evans/Christie example. The second case raises the question whether incorrectly releasing the guilty is more or less likely when the penalty is death as opposed to imprisonment and for all sorts of reasons that's almost impossible to determine with any accurately. But it depends on the psychology of the jury and that is a very complex and subtle area.