Because I don't own anything Ruth related, I'd take Ruth, but that's a personal decision.
What are you after? Long term investment or just for the sake of owning either?
I'd say the Ruth has staying/growing power whereas the Koufax could go either way.
Just noticed this thread:
http://www.net54baseball.com/showthread.php?t=261361
I'd say you likely have enough to go on now even though you now asked in the Post War section.