Very difficult to answer that one. IR pilots will tell you they are safer than IMC who will in turn say they are better/safer than basic PPL holders.
Having an instrument rating of any sort will of course give you the upper hand in marginal or IMC but come on..... theres a lot more to being a safe pilot than a certificate allowing you to fly in or through the clag.
CFIT often involves IR rated pilots getting it wrong. I took an IMC to give me more scope whilst flying in Scotland but I know plenty of basic PPL`s who are far safer and better than I am because of their greater flying experience.
Being current on your type and keeping your skill levels updated regardless of ratings would I believe make all of us safer pilots.